Fix passing nothing to start kwarg

[odow]

Came up here: https://discourse.julialang.org/t/setting-start-values-for-only-some-indicies-of-a-bileveljump-variable/81160/2

[codecov]

Codecov Report

Merging #2985 (f36faef) into master (8384a42) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2985   +/-   ##
=======================================
  Coverage   95.43%   95.43%           
=======================================
  Files          43       43           
  Lines        5803     5803           
=======================================
  Hits         5538     5538           
  Misses        265      265           

Impacted Files	Coverage Δ
src/variables.jl	98.71% <100.00%> (ø)

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