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

All modified and coverable lines are covered by tests ✅

Project coverage is 90.43%. Comparing base (622f2e3) to head (4f200a2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1146   +/-   ##
=======================================
  Coverage   90.42%   90.43%           
=======================================
  Files         147      147           
  Lines       14448    14459   +11     
=======================================
+ Hits        13065    13076   +11     
  Misses       1383     1383           

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