validation in `resample` too slow and not skipped when condition is empty

resample evaluates parameters maxna and maxna_group via reindex call with lambda function derived from lib.validate - which scales poorly and is dead slow because call cant be dispatched to built in.

Fix:

1. Skip validator call when maxna = maxna_group = None

2. Maybe implement functionality built ins and replace reindex dispatch.

added cleanup/refactoring info / benchmark usability labels

assigned to @luenensc