Tests and warnings for issue 399 (nan in table columns)

[LucaMarconato]

Closes #399

[LucaMarconato]

failing docs is due to #1045 (comment)

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (65a281e) to head (a65cc24).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/models/models.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1050      +/-   ##
==========================================
- Coverage   92.22%   92.21%   -0.01%     
==========================================
  Files          49       49              
  Lines        7585     7593       +8     
==========================================
+ Hits         6995     7002       +7     
- Misses        590      591       +1     

Files with missing lines	Coverage Δ
src/spatialdata/models/models.py	88.60% <87.50%> (-0.02%)	⬇️

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