I think the case where the array itself contains nans should also not throw a warning.

import numpy as np
import matplotlib.pyplot as plt

A = np.random.rand(4, 4)
A[:2] = np.nan

fig, ax = plt.subplots(1, 1)
im = ax.imshow(A)
fig.colorbar(im)

I don't think an API change note is needed. This simply fixes the bug introduced in #12459.
In 3.0.3 the outcome was [] as well.

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.1.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 8ed7c78725f948cf99ef65df53d4b9b23fc39d6a

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #15140: Fix ScalarFormatter formatting of masked values'

4. Push to a named branch :

git push YOURFORK v3.1.x:auto-backport-of-pr-15140-on-v3.1.x

5. Create a PR against branch v3.1.x, I would have named this PR:

"Backport PR #15140 on branch v3.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.