Fix Matpower base power parser

[rodrigomha]

Currently if data has 0 base power for units, the conversion default to NaN.

[codecov]

Codecov Report

Attention: Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.

Project coverage is 84.29%. Comparing base (91bc0ab) to head (2d4b336).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parsers/power_models_data.jl	56.52%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1218      +/-   ##
==========================================
- Coverage   84.35%   84.29%   -0.07%     
==========================================
  Files         181      181              
  Lines        8375     8391      +16     
==========================================
+ Hits         7065     7073       +8     
- Misses       1310     1318       +8     

Flag	Coverage Δ
unittests	84.29% <56.52%> (-0.07%)	⬇️

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Files with missing lines	Coverage Δ
src/parsers/power_models_data.jl	90.08% <56.52%> (-1.91%)	⬇️