REF: Simplify JoinUnit.is_na for categorical
In JoinUnit.is_na we get the categories

https://github.com/pandas-dev/pandas/blob/6cacdde5630c593999059833b516e1fec60aaf72/pandas/core/internals.py#L5782-L5783

and then check if they're all not na. Categories can't have NA in them, so we can just return False (I think).