Accept None in _XfailMarkDecorator's conditionparam and expose default value

[Avasam]

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Fixes comments pypa/setuptools#3979 (comment) and #10094 (comment)
Should not conflict with #11501

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[RonnyPfannschmidt]

I'm very torn on allowing this,
My personal preference is not to allow none there

[Avasam]

If not, then the documentation for pytest.mark.xfail should probably be updated to not use None as a default value instead.

[nicoddemus]

I agree we should probably just update the docs: the docs used None probably to mean "optional" (in the sense that might be given or not), not necessarily to accept None (similar to the changes in #11501). 👍

Thanks @Avasam I'm closing this then, would appreciate if you followed up with another PR just to update the docs.