DANFSE: corrige erro de conversão float quando pis/cofins vazios

[antoniospneto]

Resumo

Corrige ValueError: could not convert string to float: '' ao gerar DANFSe quando o elemento <piscofins> existe no XML mas as tags <vPis> e <vCofins> estão ausentes.

Substitui a #158 com:

• Fix mais enxuto (float(pis or 0) + float(cofins or 0)), sem helper novo nem try/except redundante
• Apenas os 2 PDFs afetados pela mudança de fixture foram regerados (os *_hom.pdf ficam intactos, já que a fixture de homologação não muda)
• PDFs regerados em ambiente alinhado ao CI (fpdf2 2.8.7, Pillow 12.2.0, fonttools 4.63.0)

Fecha #158.

Co-authored-by: @leogregianin

Test plan

• pytest tests/test_danfse.py passa localmente com qpdf instalado (mesma comparação do CI)
• Suite completa passa (58 testes, ignorando test_cli.py por dep ausente local)
• CI verde

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.65%. Comparing base (32e6810) to head (20e994f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
+ Coverage   95.49%   95.65%   +0.15%     
==========================================
  Files          32       32              
  Lines        4393     4393              
  Branches      311      311              
==========================================
+ Hits         4195     4202       +7     
+ Misses        131      125       -6     
+ Partials       67       66       -1     

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