nan handling for continuous and categorical coloring

[Sonja-Stockhaus]

Fix coloring of shapes, labels and points when there are nan values in the vectors to color by

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.00000% with 14 lines in your changes missing coverage. Please review.

Project coverage is 84.45%. Comparing base (478ab27) to head (da69a42).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/render.py	60.00%	12 Missing ⚠️
src/spatialdata_plot/pl/utils.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   83.83%   84.45%   +0.62%     
==========================================
  Files           8        8              
  Lines        1732     1795      +63     
==========================================
+ Hits         1452     1516      +64     
+ Misses        280      279       -1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	78.40% <80.00%> (+0.39%)	⬆️
src/spatialdata_plot/pl/render.py	92.61% <60.00%> (+1.45%)	⬆️

... and 1 file with indirect coverage changes

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