Add unit test for all functions

[lovung]

No description provided.

[codecov-io]

Codecov Report

Merging #9 (2c74002) into main (0f6c730) will increase coverage by 38.33%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main       #9       +/-   ##
===========================================
+ Coverage   48.88%   87.22%   +38.33%     
===========================================
  Files          11       11               
  Lines         180      180               
===========================================
+ Hits           88      157       +69     
+ Misses         91       23       -68     
+ Partials        1        0        -1     

Impacted Files	Coverage Δ
nulldate/null_date.go	100.00% <0.00%> (+78.12%)	⬆️
util.go	100.00% <0.00%> (+100.00%)	⬆️
nulldate/sql.go	100.00% <0.00%> (+100.00%)	⬆️

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