Redraw empty histogram layer

[Carifio24]

This PR addresses an issue that was discussed at a glue meeting a little while ago (don't remember the exact date). The issue is that if a subset group is modified to make it empty, the corresponding histogram layer artist is not redrawn. See the video below.

histogram-bug.mp4

The issue seems to be here, where _update_artists return if the sum of the histogram is empty (that is, the layer has no content). To fix this, I've removed the mpl_hist.sum() == 0 check. Allowing the possibility for empty layer opens the door for the largest_y_max and smallest_y_min values computed later in the method to be NaN or infinite (in particular, if all of the layers are empty). I couldn't think of a better default behavior than just punting on updating the bounds in this case, so I've added numpy.isfinite checks around both assignments to the relevant viewer state properties.

[codecov]

Codecov Report

Merging #2300 (dd82bc8) into main (e1331ec) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #2300      +/-   ##
==========================================
- Coverage   88.14%   88.11%   -0.04%     
==========================================
  Files         247      247              
  Lines       23365    23369       +4     
==========================================
- Hits        20596    20591       -5     
- Misses       2769     2778       +9     

Impacted Files	Coverage Δ
glue/viewers/histogram/state.py	92.15% <0.00%> (-2.48%)	⬇️
glue/viewers/histogram/layer_artist.py	93.04% <100.00%> (-0.87%)	⬇️
glue/conftest.py	63.09% <0.00%> (-4.77%)	⬇️

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[astrofrog]

@Carifio24 - this looks good! Could you add a test and changelog entry?

[Carifio24]

@astrofrog I've added a changelog entry and a test (TestHistogramViewer::test_redraw_empty_subset) that checks whether all of the Rectangles in the layer artist have height zero after the subset state is set to an empty one.

[astrofrog]

Looks good, thanks!