All Swift events should not have `NOT_GRB` label

Currently every Swift event after !730 (merged) has been incorrectly given the NOT_GRB label, which is only intended if Swift loses lock (StarTrack_Lost_Lock).

This error is occurring because the value ('false') parsed from the VOEvent is a string so returns true (i.e. if 'false' => True).

See example event: https://gracedb.ligo.org/events/E365854/view/