Currently, if we create a category in ObjC that is empty, we still emit runtime metadata for that category. This is a scenario that could commonly be run into when using __attribute__((objc_direct_members)), which elides the need for much of the category metadata. This is slightly wasteful and can be easily skipped by checking the category metadata contents during CodeGen.
rdar://66177182
Does this really need to be an &=? Is there any case where isEmptyCategory isn't true here?