fix plot categorical with colors, on labels

[LucaMarconato]

When plotting a categorical obs column on labels, for which colors were present in the anndata, an error occurred. This was because the variable color_dict, which should be a dict, was an array of hex strings instead.

This part of the code is not covered by tests; we should have tests like in spatialdata-plot, but since we are going to work on visualization interoperability, it makes sense to wait and have a more general way to test the same plots across all the viewers without having to manually duplicate the code and maintain redundant tests. CC @melonora