fix: only validate colors if the key is specified

[joyceyan]

Reason for Change

• based on feedback here and here, it looks like we should only do validation on these fields if the key is specified in uns

Changes

• updating the validation to only run the validation on the colors fields if the key is set and the value is not an empty array

Testing

• added a test to verify that if uns["suspension_colors"] is removed, we'll still pass validation

Notes for Reviewer

[codecov]

Codecov Report

Merging #647 (43d0e5e) into main (40d56c8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 43d0e5e differs from pull request most recent head 1aa127d. Consider uploading reports for the commit 1aa127d to get more accurate results

@@           Coverage Diff           @@
##             main     #647   +/-   ##
=======================================
  Coverage   83.70%   83.71%           
=======================================
  Files          19       19           
  Lines        1737     1738    +1     
=======================================
+ Hits         1454     1455    +1     
  Misses        283      283           

Flag	Coverage Δ
unittests	83.71% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
cellxgene_schema_cli/cellxgene_schema/validate.py	94.61% <100.00%> (+<0.01%)	⬆️

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