REF: Rework core.dtypes.cast.maybe_promote

[mattayes]

Problem description

xref #18313

note if you are interested in cleaning some things up :>

maybe_promote I think will pretty much ignore its fill value for almost all dtypes. So the question is, when is it not ignored, can we safely remove it (or really just make the default fill_value=None, and then when its not None actually do something.

Not sure of the answer here.

[mattayes]

@jreback Could you expand on the options you're considering?

[jreback]

see how/when maybe_promote is called. I suspect we don't actually use the passed in fill arg. If so we can dispense with it. but maybe I am wrong and it is used. In that case, the default should be fill_value=None, just makes it more clear.

[jbrockmendel]

Closed by #28913