Make colorbars constructible with dataless ScalarMappables. - #12767

This allows creating "bare" colorbars with the one-liner

fig.colorbar(cm.ScalarMappable(norm=norm, cmap=cmap))

xref #3644 (comment)

As to the answer to the question in that comment ("What would be the implication if that is changed to self._A = []?"):

That wouldn't work because ScalarMappables need to also support cases where the color is directly set by the end user (explicit RGBA image, or explicit colors in scatter plots), in which case they mark that fact with A == None (at least initially, then A becomes the RGBA array, the whole thing looks a bit messy...).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way