Fix issue when headers are parsed and a NaN is encountered

[c-randall]

Description

For excel files with NaN in a given row the find_headers function would break because even when the row is converted to a str, the NaN remains a float. This has now been fixed by replacing NaN with empty strings during header parsing.

This PR also includes some minor changes to the error codes that ruff checks for.

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that improves speed/readability/etc.)
• Bug fix (non-breaking change which fixes an issue)
• Chore (any other clean up, maintenance, etc.)

Key checklist:

• No style issues: $ nox -s linter [-- format]
• Code is free of misspellings: $ nox -s codespell [-- write]
• All tests pass: $ nox -s tests
• Badges are updated: $ nox -s badges

Further checks:

• The documentation builds: $ nox -s docs.
• Code is commented, particularly in hard-to-understand areas.
• Tests are added that prove fix is effective or that feature works.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.07%. Comparing base (8dcfec8) to head (a9c5a83).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ampworks/_core/_read.py	25.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   30.09%   30.07%   -0.02%     
==========================================
  Files          40       40              
  Lines        1738     1739       +1     
==========================================
  Hits          523      523              
- Misses       1215     1216       +1     

Flag	Coverage Δ
unittests	30.07% <25.00%> (-0.02%)	⬇️

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