playback: Only send a packet to the audio backend if it isn't empty
playback: Only send a packet to the audio backend if it isn't empty

The lewton decoder sometimes delivers empty packets, especially after skipping inside a
track or switching tracks. This caused the pulseaudio backend to fail since it expects
a non-empty packet. There is no need to handle empty packets in the audio backend, so
we can skip them entirely.

michaelherger/librespot,michaelherger/librespot