Add check to see if cmap is a tuple of hex values

[hoxbro]

The problem was first observed here https://github.com/holoviz/holoviews/pull/5607

Todo

• [x] Add a unit test

[codecov[bot]]

Codecov Report

Merging #1173 (08ff7b0) into main (35fff08) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1173   +/-   ##
=======================================
  Coverage   85.36%   85.36%           
=======================================
  Files          35       35           
  Lines        8008     8010    +2     
=======================================
+ Hits         6836     6838    +2     
  Misses       1172     1172           

Impacted Files	Coverage Δ
datashader/transfer_functions/__init__.py	86.86% <100.00%> (+0.04%)	:arrow_up:

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[ianthomas23]

The pip test failures are unrelated.

[ianthomas23]

Test could go in test_transfer_functions.py near test_shade_cmap() and similar.