MimeBundleFormatter.__call__ should not return None. #10650

__call__ seem to be expected to always return a tuple in IPython.core.display:display

Moreover the formatter take precedence over the user registered formatter, while the _repr_*_ do not.

I believe this is wrong, as user registered formatter should take precedence.
An option for user register formatter to not take precedence (and opt-in) would make sens for things like ipython/disp.