Only use long_name if it is a string

[hoxbro]

Fixes #5615

@jlstevens I know we discussed having a warning saying that the long name should be a string, but coming back to the code after a week, I think I would rather ignore the long name if it is not a string.

[jlstevens]

I think that is another very reasonable policy.

[codecov-commenter]

Codecov Report

Merging #5646 (08d44b7) into main (eeabab2) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5646      +/-   ##
==========================================
+ Coverage   88.25%   88.32%   +0.06%     
==========================================
  Files         302      302              
  Lines       62532    62579      +47     
==========================================
+ Hits        55189    55274      +85     
+ Misses       7343     7305      -38     

Impacted Files	Coverage Δ
holoviews/core/data/xarray.py	88.86% <100.00%> (ø)
...oloviews/tests/plotting/bokeh/test_quadmeshplot.py	100.00% <0.00%> (ø)
holoviews/tests/plotting/bokeh/test_renderer.py	100.00% <0.00%> (+1.20%)	⬆️
holoviews/plotting/bokeh/util.py	64.95% <0.00%> (+1.82%)	⬆️
holoviews/plotting/bokeh/raster.py	83.33% <0.00%> (+11.71%)	⬆️

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[hoxbro]

Great to hear will merge this then.