Should ExtensionBlock.fillna unbox Series / Index?
https://github.com/pandas-dev/pandas/blob/9caf04836ad34ca17da7b86ba7120cca58ce142a/pandas/core/internals/blocks.py#L2056

Should we get the ExtensionArray / ndarray backing the Series / Index at that point? As an EA author, I'd rather not have to worry about extracting the extension array from a Series in my fillna.

In the ExtensionArray.fillna docstring, we say the type can be 'arraylike'.