Fix serialization issue in XASDoc

[esoteric-ephemera]

Adds a field_validator to XASDoc to check that the spectrum only has positive intensities (related pymatgen PR). If a spectrum has non-positive intensities, they are set to a small positive value (like zeroing out, but needs to be positive in this case).

Also adds a test for the new behavior (previously no tests for XASDoc)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.94%. Comparing base (a705a61) to head (7846e7d).

Files with missing lines	Patch %	Lines
emmet-core/emmet/core/xas.py	46.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1146      +/-   ##
==========================================
- Coverage   90.42%   86.94%   -3.49%     
==========================================
  Files         147       83      -64     
  Lines       14448    11614    -2834     
==========================================
- Hits        13065    10098    -2967     
- Misses       1383     1516     +133     

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