REF: Simplify JoinUnit.is_na for categorical

[TomAugspurger]

In JoinUnit.is_na we get the categories

pandas/pandas/core/internals.py

Lines 5782 to 5783 in 6cacdde

	if self.block.is_categorical:
	values_flat = values.categories

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

[jbrockmendel]

the categories cant have NAs, but the codes can have -1s