Fixes matplotlib empty BoxWhisker plot

[hoxbro]

Fixes #5120

[codecov-commenter]

Codecov Report

Merging #5450 (67356d6) into master (d8b83d1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5450      +/-   ##
==========================================
+ Coverage   88.09%   88.11%   +0.02%     
==========================================
  Files         301      301              
  Lines       61990    61969      -21     
==========================================
- Hits        54608    54607       -1     
+ Misses       7382     7362      -20     

Impacted Files	Coverage Δ
holoviews/plotting/mpl/stats.py	87.17% <100.00%> (+0.11%)	⬆️
holoviews/plotting/util.py	70.72% <0.00%> (-0.16%)	⬇️
holoviews/tests/test_streams.py	98.24% <0.00%> (-0.11%)	⬇️
holoviews/plotting/bokeh/element.py	89.36% <0.00%> (+0.50%)	⬆️
holoviews/tests/plotting/bokeh/test_callbacks.py	100.00% <0.00%> (+1.37%)	⬆️
holoviews/tests/core/data/test_daskinterface.py	97.01% <0.00%> (+10.34%)	⬆️

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[github-actions]

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