showing a doc string in the help mode of the REPL may throw in highlighting on nightly Julia

[nsajko]

Possible JuliaSyntaxHighlighting.jl bug, move if appropriate, cc @tecosaur

Showing the Base.showarg help throws:

help?> Base.showarg
[...]
  The default show printing would display this full type. However, the summary
  for SubArrays actually prints as

Error showing value of type Markdown.MD:
ERROR: MethodError: no method matching keys(::Nothing)
The function `keys` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  keys(::Core.SimpleVector)
   @ Base essentials.jl:945
  keys(::Cmd)
   @ Base process.jl:716
  keys(::Base.TermInfo)
   @ Base terminfo.jl:232
  ...

Stacktrace:
  [1] pairs(collection::Nothing)
    @ Base ./abstractdict.jl:178
  [2] findfirst(testf::JuliaSyntaxHighlighting.var"#17#18", A::Nothing)
    @ Base ./array.jl:2433
  [3] _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int64}, label::Symbol, value}}, lineage::JuliaSyntaxHighlighting.GreenLineage, ctx::JuliaSyntaxHighlighting.HighlightContext{String}; syntax_errors::Bool)
    @ JuliaSyntaxHighlighting ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:222
  [4] _hl_annotations!
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:181 [inlined]
  [5] _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int64}, label::Symbol, value}}, lineage::JuliaSyntaxHighlighting.GreenLineage, ctx::JuliaSyntaxHighlighting.HighlightContext{String}; syntax_errors::Bool)
    @ JuliaSyntaxHighlighting ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:343
  [6] _hl_annotations!
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:181 [inlined]
  [7] _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int64}, label::Symbol, value}}, lineage::JuliaSyntaxHighlighting.GreenLineage, ctx::JuliaSyntaxHighlighting.HighlightContext{String}; syntax_errors::Bool)
    @ JuliaSyntaxHighlighting ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:343
  [8] _hl_annotations!
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:181 [inlined]
  [9] #_hl_annotations#10
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:170 [inlined]
 [10] _hl_annotations
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:167 [inlined]
 [11] highlight
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:392 [inlined]
 [12] highlight
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/JuliaSyntaxHighlighting/src/JuliaSyntaxHighlighting.jl:383 [inlined]
 [13] term(io::IOContext{REPL.LimitIO{Base.TTY}}, md::Markdown.Code, columns::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:120
 [14] term(io::IOContext{REPL.LimitIO{Base.TTY}}, content::Vector{Any}, cols::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:10
 [15] term(io::IOContext{REPL.LimitIO{Base.TTY}}, md::Markdown.MD, columns::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:18
 [16] term(io::IOContext{REPL.LimitIO{Base.TTY}}, content::Vector{Any}, cols::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:13
 [17] term(io::IOContext{REPL.LimitIO{Base.TTY}}, md::Markdown.MD, columns::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:18
 [18] term(io::IOContext{REPL.LimitIO{Base.TTY}}, content::Vector{Any}, cols::Int64)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:13
 [19] term
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:18 [inlined]
 [20] term
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:17 [inlined]
 [21] show(io::IOContext{REPL.LimitIO{Base.TTY}}, ::MIME{Symbol("text/plain")}, md::Markdown.MD)
    @ Markdown ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/Markdown/src/render/terminal/render.jl:231
 [22] show_repl(io::IO, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:574
 [23] show_limited(io::IO, mime::MIME, x::Any)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:543
 [24] (::REPL.var"#display##0#display##1"{REPL.REPLDisplay{REPL.LineEditREPL}, MIME{Symbol("text/plain")}, Base.RefValue{Any}})(io::Any)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:566
 [25] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:867
 [26] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:552
 [27] display
    @ ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:572 [inlined]
 [28] display(x::Any)
    @ Base.Multimedia ./multimedia.jl:340
 [29] #invokelatest#1
    @ ./essentials.jl:1057 [inlined]
 [30] invokelatest
    @ ./essentials.jl:1053 [inlined]
 [31] print_response(errio::IO, response::Any, show_value::Bool, have_color::Bool, specialdisplay::Union{Nothing, AbstractDisplay})
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:615
 [32] (::REPL.var"#print_response##0#print_response##1"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:584
 [33] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:867
 [34] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:582
 [35] (::REPL.var"#do_respond#71"{Bool, Bool, REPL.var"#setup_interface##0#setup_interface##1"{REPL.LineEditREPL}, REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:1209
 [36] #invokelatest#1
    @ ./essentials.jl:1057 [inlined]
 [37] invokelatest
    @ ./essentials.jl:1053 [inlined]
 [38] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/LineEdit.jl:2852
 [39] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:1680
 [40] (::REPL.var"#59#60"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL ~/tmp/jl/jl/nightly/assert/julia-7e6971024b/share/julia/stdlib/v1.13/REPL/src/REPL.jl:686

[tecosaur]

Yea, I've been needing to find the time to actually review a PR that will fix this along the way. Fingers crossed I'll actually be able to do this in the next day or few and not have a bunch of stuff come up ...again.

This was raised earlier in #57402, which I think this is a pretty clear duplicate of.