Notifications: Do not play sound if message has more than one recipient

This prevents Gajim from playing the message_sent sound multiple times
3 jobs for sound-notification-multiple-contacts in 3 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #11015
run-appdata

00:00:08

passed #11014
run-tests

00:03:21

 
  Build
passed #11016
run-build

00:00:13