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

All modified and coverable lines are covered by tests ✅

Project coverage is 85.32%. Comparing base (667f430) to head (82d6d49).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   84.92%   85.32%   +0.40%     
==========================================
  Files           8        8              
  Lines        1784     1820      +36     
==========================================
+ Hits         1515     1553      +38     
+ Misses        269      267       -2     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	97.14% <100.00%> (+0.76%)	⬆️
src/spatialdata_plot/pl/utils.py	78.25% <100.00%> (+0.06%)	⬆️

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[Sonja-Stockhaus]

In seg, the value 0 depicts the background, so this would lead to the bg being mapped to the NaN color
The actual label(s) with na in the color_source_vector don't have their id in cell_id anymore, so they're mapped to nothing! => would look like background