Add None as possible parameter value to isnull/isna/notnull/notna

[gandhis1]

• Closes pd.isna / pd.isnull raising mypy error for None #318
• Tests added: Please use assert_type() to assert the type of any return value

[marctorsoc]

I wrote exactly the same changes as you and was going to create a PR when I saw this 🤣 good job!

[Dr-Irv]

thanks @gandhis1

[gandhis1]

One additional thought here - is there any way we can make these functions a TypeGuard so they can be used to narrow types? Right now it will not narrow an Optional or a Union[np.nan, T], but it should.

[Dr-Irv]

One additional thought here - is there any way we can make these functions a TypeGuard so they can be used to narrow types? Right now it will not narrow an Optional or a Union[np.nan, T], but it should.

Open to a PR that does this. Would have to import TypeGuard from typing_extensions