To solve the underlying issue of reindexing with a fill value on an extension array (the test added is a reproducer), we special case nan values for type promotion because they are native pandas object that should be handled by the underlying extension arrays, which have a concept of nans in general (i.e., not specific to one kind of extension array, see: https://pandas.pydata.org/docs/reference/api/pandas.api.extensions.ExtensionArray.fillna.html)

Note this PR does not change the behavior otherwise - we still disallow categoricals, for example, from being collectively type-promoted. That could come in a separate PR, but would require further special casing i.e., for #10247

cc @ilia-kats

• Catches a use case from RecursionError when reindexing Dataset with pandas extension arrays inside #10218 where a fill value is provided.
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst