[Bug]: False positive legend handler warnings in 3.6.0.rc1

[mwaskom]

Bug summary

With the 3.6.0.rc1, matplotlib is issuing warnings when preparing a legend if the axes contains artists that do not have a registered legend handler even if the artist was not provided with a label.

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

f, ax = plt.subplots()
ax.pcolormesh(np.random.uniform(0, 1, (10, 10)))
ax.get_legend_handles_labels()

Actual outcome

Warning:

/var/folders/pk/kq0vw6sj3ssd914z55j1qmzc0000gn/T/ipykernel_68178/1573363929.py:6: UserWarning: Legend does not support handles for QuadMesh instances.
See: https://matplotlib.org/stable/tutorials/intermediate/legend_guide.html#implementing-a-custom-legend-handler
  ax.get_legend_handles_labels()

Expected outcome

This was not previously so noisy.

Additional information

I skimmed the existing issues and didn't see anything obviously reporting this, but haven't tried against main so apologies if this is a dupe.

Matplotlib Version

3.6.0.rc1

[jklymak]

This was broken by #20470. This check should be bypassed if label is '' or 'nolegend', but for some reason the label here is _child0, which is done in ax.add_collection. I think the fix is straight forwardly to skip any legends that start with _ in this check.

[jklymak]

If you have the inclination, you could make sure #23762 fixes it in your real code...