app_queue: Fix returning to dialplan when a queue is empty

The fix for ASTERISK-25665 introduced a regression.
The return value of queue_exec used to be 0 in case of leavewhenempty
but it was changed to -1 (returned from wait_our_turn and passed
transparently by queue_exec), thus leading to hangup instead of returning
back to dialplan.

This commit resets the value back to 0 in this case, restoring
original behavior.

ASTERISK-27065 #close
Reported by: Marek Cervenka

Change-Id: Id9c83b75aeda463250155e88c5004be52bbca5ac