Add set_start_value(x, ::Nothing)

[odow]

Closes #2237

[codecov]

Codecov Report

Merging #2551 (050eb61) into master (d9588b4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2551   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files          44       44           
  Lines        5456     5457    +1     
=======================================
+ Hits         5071     5072    +1     
  Misses        385      385           

Impacted Files	Coverage Δ
src/variables.jl	98.78% <100.00%> (+<0.01%)	⬆️

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