snd_worker: Don't send empty audio-volume messages
snd_worker: Don't send empty audio-volume messages

If no volume has been set it, we end up sending a volume message with
audio-volume for 0 channels (iow an empty message). This is not useful
and triggers the following warning in spice-gtk:

(remote-viewer:8726): GSpice-WARNING **: set_sink_input_volume() failed:
Invalid argument

Signed-off-by: Hans de Goede <9fa1be1a5b5729e4c6b404f34c9ce49ff4882fd8@redhat.com>

fgouget/spice,Fantu/spice,fgouget/spice,fgouget/spice,alon/spice-server,ya-mouse/spice,alon/spice-server,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,elmarco/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,Fantu/spice,elmarco/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,alon/spice-server,ya-mouse/spice,Fantu/spice,SPICE/spice,alon/spice-server,ya-mouse/spice,SPICE/spice