Fix: deserialize nullability

[kitsuyui]

• Some fields are nullable, but the deserializer was not handling that
  correctly. This fixes that.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6a436d9) 95.18% compared to head (ba90eb1) 95.28%.
Report is 13 commits behind head on main.

Files	Patch %	Lines
src/branch_detail.rs	98.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   95.18%   95.28%   +0.09%     
==========================================
  Files           9        9              
  Lines         395      445      +50     
==========================================
+ Hits          376      424      +48     
- Misses         19       21       +2     

Flag	Coverage Δ
unittests	95.28% <98.52%> (+0.09%)	⬆️

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