Without this fix, hovering the mouse over a nan pixel in an imshow()
would result in Warning: converting a masked element to nan. and then
ValueError: cannot convert float NaN to integer. Fix that.

The format_cursor_data test doesn't explicitly check the return value
of get_cursor_data anymore because np.ma.masked is returned for nan
inputs (this is expected from the general approach for handling invalid
data); checking that format_cursor_data gives the right string is
sufficient.

Regression coming from #20949.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).