Set default searches in coincidence search to empty list
This error occurs when coincidence searches with SNEWS events are called, which don't specify the search
field (see: https://git.ligo.org/brandon.piotrzkowski/gwcelery/-/blob/7740f6a082d70310c3ad46e09c054f748fe46d03/gwcelery/tasks/external_triggers.py#L183-190) and therefore currently use the default None
which is no longer an acceptable argument.
This fixes the sentry error: https://sentry.io/organizations/ligo-caltech/issues/2957802469/?project=1425216&query=is%3Aunresolved&statsPeriod=14d