BUG: Groupby.filter with pd.NA
@Dr-Irv this is an example of the kind of bug that I have in mind when I said in the Categorical issue that getting pd.NA is a PITA.   We've had 3 years to track them down and we're still finding places where having it unexpectedly causes bugs.  Users will have this long tail until the end of time.
> @Dr-Irv this is an example of the kind of bug that I have in mind when I said in the Categorical issue that getting pd.NA is a PITA. We've had 3 years to track them down and we're still finding places where having it unexpectedly causes bugs. Users will have this long tail until the end of time.

That's a bit unavoidable at this point, unless we decide to get rid of `pd.NA` , but I don't see that happening
 Is there a potential performance impact? > Is there a potential performance impact?

Hard to see how

 My thinking was notna might be more expensive than checking b first thx @jbrockmendel 