Return early in eq_hist() if all data masked out

[ianthomas23]

Fixes #1166.

If pass an fully-masked array to eq_hist() function, return early with an array of NaNs. This avoids calculating the cumulative distribution function of no data and the resultant IndexError because the CDF has no contents.

Passes new test locally on both CPU and GPU.

[codecov[bot]]

Codecov Report

Merging #1168 (4398ff9) into main (73d3deb) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1168   +/-   ##
=======================================
  Coverage   85.40%   85.40%           
=======================================
  Files          35       35           
  Lines        7987     7989    +2     
=======================================
+ Hits         6821     6823    +2     
  Misses       1166     1166           

Impacted Files	Coverage Δ
datashader/transfer_functions/__init__.py	86.81% <100.00%> (+0.04%)	:arrow_up:

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