sound: Do not return an error from chn_notify() if the children list is empty

An empty children list does not mean that there is an error.

Reported by: Florian Walpen <dev@submerge.ch>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48156

(cherry picked from commit 164651f16c860049a4ad2098206def8ca7a39b4a)