Fix issue when an empty palette is provided for an empty dataset

[thuiop]

Fixes issue #3118, by checking whether self._hue_palette.levels is None before calling len on it.

[codecov]

Codecov Report

Merging #3136 (8586bb7) into master (d4d27ad) will not change coverage.
The diff coverage is 100.00%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3136   +/-   ##
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  Coverage   98.41%   98.41%           
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  Files          76       76           
  Lines       24076    24076           
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  Hits        23695    23695           
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/categorical.py	98.54% <100.00%> (ø)

[mwaskom]

As mentioned in the issue, I think that most functions short-circuit when there's no x/y data to avoid the need for this downstream special-casing.

This seems fine, but for it to be accepted, there would need to be a test replicating the original problem.

[thuiop]

Actually it seems that only categorical plots would be concerned by the issue, as in the other cases len is never called on self._hue_map.levels. Anyway, I added a small test and checked that it failed before this PR and is now passing.