Add support for string types to isnull

[jpdehollain]

• Closes combine_first doesn't fill empty strings #10280
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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[jpdehollain]

I left all the checkboxes in the description because I'm not 100% sure which ones apply to this. Let me know if I need to address any of the open ones.

[jpdehollain]

Closing as the implemented changes are not compatible with many other functions