res_stasis: Don't unsubscribe from a NULL bridge.

A NULL bridge has special meaning in res_stasis for
unsubscribing. It means that a subscription to ALL
bridges should be removed. This should not be done
as part of the normal subscription management in
the res_stasis channel loop.

ASTERISK-26468

Change-Id: I6d5bea8246dd13a22ef86b736aefbf2a39c15af0