fix nil valuer #255

[benzolium]

I've just copy-pasted #256 but added a simple test. Feel free to copypaste my test and merge original solution.

[benzolium]

@doug-martin, could you please take a look? I still have to use fork. :(

[doug-martin]

Closed and reopened to run tests

[codecov]

Codecov Report

Merging #277 (ee6a5ed) into master (ff08a24) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #277   +/-   ##
=======================================
  Coverage   96.77%   96.78%           
=======================================
  Files          60       60           
  Lines        3319     3324    +5     
=======================================
+ Hits         3212     3217    +5     
  Misses         91       91           
  Partials       16       16           

Flag	Coverage Δ
unittests	96.78% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sqlgen/expression_sql_generator.go	100.00% <100.00%> (ø)

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