write all NaN and NaT Dataframe created values as null

[kbuma]

Summary

Some collections do not have the same set of fields present in all the documents. The underlying DataFrames implementation for OpenData store uses NaN and NaT to fill in missing values for these fields. This is incompatible with Mongo storage (no NaT) and our JSON serialization. This adds code to convert NaN and NaT values to None prior to writing the documents out to S3 in OpenDataStore.

Checklist

• Google format doc strings added.
• Code linted with ruff. (For guidance in fixing rule violates, see rule list)
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• I have run the tests locally and they passed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 81.52%. Comparing base (f5f5593) to head (33806c1).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/maggma/stores/open_data.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #929      +/-   ##
==========================================
- Coverage   81.56%   81.52%   -0.05%     
==========================================
  Files          46       46              
  Lines        3938     3940       +2     
==========================================
  Hits         3212     3212              
- Misses        726      728       +2     

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