j auto-naming might delete an explicit NULL

[MichaelChirico]

Closes #4061

explicit NULL was leading essentially to:

q[[n]] = NULL # <- deletes q[[n]]

and then the problem.

This issue was already handled in the if branch -- skip assignment if the object is NULL.

[codecov]

Codecov Report

Merging #4063 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4063      +/-   ##
=========================================
+ Coverage    99.4%   99.4%   +<.01%     
=========================================
  Files          72      72              
  Lines       13674   13675       +1     
=========================================
+ Hits        13593   13594       +1     
  Misses         81      81

Impacted Files	Coverage Δ
R/data.table.R	100% <100%> (ø)	⬆️
src/fread.c	99.44% <0%> (ø)	⬆️
src/fwrite.c	97.87% <0%> (ø)	⬆️
src/fsort.c	95.83% <0%> (+0.02%)	⬆️

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