notifications: Bail if we can't find an OK parent

If all of a service's parents are in a non-OK state, we shouldn't send
out notifications.

Reported-by: Sven Velt <sven@velt.de>
Original-patch-by: Sven Velt <sven@velt.de>
Signed-off-by: Anton Lofgren <alofgren@op5.com>