Run the downstream playback queue even if there are no samples
in the mixing engine buffer. The downstream queue may still have
queued samples.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 20200920171729.15861-7-vr_qemu@t-online.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>