add Null case test for literal.rs

[hyangmin82]

No description provided.

[codecov-commenter]

Codecov Report

Merging #412 (6b5f32b) into main (186b689) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #412   +/-   ##
=======================================
  Coverage   90.39%   90.40%           
=======================================
  Files         144      144           
  Lines        8539     8543    +4     
=======================================
+ Hits         7719     7723    +4     
  Misses        820      820           

Impacted Files	Coverage Δ
src/data/literal.rs	83.69% <100.00%> (+0.28%)	⬆️

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