feat: checks whether g type is null

[PokIsemaine]

fix: #64

[casbin-bot]

@tangyang9464 @imp2002 please review

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0119b27) 73.33% compared to head (6c605a0) 73.33%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   73.33%   73.33%           
=======================================
  Files           3        3           
  Lines          90       90           
  Branches       26       26           
=======================================
  Hits           66       66           
  Misses         13       13           
  Partials       11       11           

Files Changed	Coverage Δ
.../main/java/org/casbin/adapter/JDBCBaseAdapter.java	100.00% <ø> (ø)

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