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

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 85.32%. Comparing base (667f430) to head (82d6d49). :warning: Report is 14 commits behind head on main.

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%)	:arrow_up:
src/spatialdata_plot/pl/utils.py	78.25% <100.00%> (+0.06%)	:arrow_up:

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