Fix issue in CSV.write for empty strings; fixes #515

[quinnj]

No description provided.

[codecov]

Codecov Report

Merging #516 into master will increase coverage by 11.44%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #516       +/-   ##
===========================================
+ Coverage   72.34%   83.79%   +11.44%     
===========================================
  Files           7        7               
  Lines        1179     1395      +216     
===========================================
+ Hits          853     1169      +316     
+ Misses        326      226      -100

Impacted Files	Coverage Δ
src/write.jl	85.14% <100%> (+3.44%)	⬆️
src/detection.jl	88.76% <0%> (+3.04%)	⬆️
src/iteration.jl	45% <0%> (+3.97%)	⬆️
src/rows.jl	84.15% <0%> (+6.38%)	⬆️
src/tables.jl	90.54% <0%> (+6.9%)	⬆️
src/utils.jl	76.13% <0%> (+13.31%)	⬆️
src/CSV.jl	85.23% <0%> (+19.29%)	⬆️

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