Add IsNullCallbackArg

[andrzh]

It took some digging to find out that NULL is passed into custom functions as a nil byte slice:

go-sqlite3/callback.go

Line 229 in f08f1b6

// Interpret NULL as a nil byte slice.

It would be nice to have this function s.t. future callers just need to call IsNullCallbackArg to determine whether the argument to their function is null.

Also, some minor auto-formatting.

[codecov-commenter]

Codecov Report

Patch coverage: 60.00% and project coverage change: +0.52 🎉

Comparison is base (f08f1b6) 46.72% compared to head (274eefa) 47.25%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1170      +/-   ##
==========================================
+ Coverage   46.72%   47.25%   +0.52%     
==========================================
  Files          12       12              
  Lines        1526     1528       +2     
==========================================
+ Hits          713      722       +9     
+ Misses        671      663       -8     
- Partials      142      143       +1     

Impacted Files	Coverage Δ
sqlite3.go	52.86% <ø> (ø)
callback.go	62.31% <60.00%> (+3.78%)	⬆️

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