Add `IsNullCallbackArg`

[andrzh]

It took some digging to find out that NULL is passed into custom functions as a nil byte slice: https://github.com/mattn/go-sqlite3/blob/f08f1b6b9ce62b2496d8d64df26c1e278887bc1c/callback.go#L229

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 :tada:

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

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

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%)	:arrow_up:

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.