Drop empty annotation rows from plot_da_beeswarm

[Zethson]

Closes #1076.

The row order was derived from nhood_adata.obs before the "nan" annotations were filtered out and with the pandas default of observed=False, so annotation categories without any neighbourhood were drawn as empty rows.
The order now comes from the plotted frame with observed=True, which also makes the plot independent of the pandas version -- the default flipped to True in pandas 3.0, so this only shows up for users on pandas 2.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.45%. Comparing base (0c429c5) to head (8944ed1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1078      +/-   ##
==========================================
- Coverage   79.46%   79.45%   -0.02%     
==========================================
  Files          53       53              
  Lines        7545     7546       +1     
==========================================
  Hits         5996     5996              
- Misses       1549     1550       +1     

Files with missing lines	Coverage Δ
src/pertpy/tools/_milo.py	78.24% <ø> (ø)

... and 1 file with indirect coverage changes

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