feat(mapper): clean up unnecessary arc workarounds

Make use of the NS_VALID_UNTIL_END_OF_SCOPE macro to make a variable stay until the end of the scope instead of using the object at the end of a method to make it stay around.
feat(mapper): make specifying a nil predicate a defined api use

This changes the implementation as suggested by lla01: When calling -setConditionForIndexPath:predicate:evaluationObject: with a nil predicate, the condition for this index path is removed.