Discard share notification for non-existing groups
Discard share notification for non-existing groups

Group shares might exist even after a group got deleted.

This fix catches the situation and discards the notification for the
obsolete group.

Signed-off-by: Vincent Petry <87e1f221a672a14a323e57bb65eaea19d3ed3804@nextcloud.com>
Co-authored-by: Joas Schilling <ab43a7c9cb5b2380afc4ddf8b3e2583169b39a02@schilljs.com>

nextcloud/server,nextcloud/server,nextcloud/server,nextcloud/server