PR Summary

Deal with imshowing an array that is completely masked without emitting a warning. The added test/code emits 3 warnings on master, none of which are particularly informative. The test on master fails with:

lib/matplotlib/image.py:639: in draw
    im, l, b, trans = self.make_image(
lib/matplotlib/image.py:924: in make_image
    return self._make_image(self._A, bbox, transformed_bbox, clip,
lib/matplotlib/image.py:476: in _make_image
    vrange = np.array([vmin, vmax], dtype=scaled_dtype)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = masked

    def __float__(self):
        """
        Convert to float.
    
        """
        if self.size > 1:
            raise TypeError("Only length-1 arrays can be converted "
                            "to Python scalars")
        elif self._mask:
>           warnings.warn("Warning: converting a masked element to nan.", stacklevel=2)
E           UserWarning: Warning: converting a masked element to nan.

And on latest NumPy it also emits a warning later having to do with np.clip not liking np.nan values.

Alternatively, imshow with all data masked could emit a warning, but it seems like the existing code had a chance to do that but didn't (there is an existing if a_min is np.ma.masked on line 409) so I didn't implement it here.

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 pydocstyle<4 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).