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.

playback/src/player.rs

@@ -372,6 +372,7 @@ impl PlayerInternal {
     fn handle_packet(&mut self, packet: Option<VorbisPacket>, normalisation_factor: f32) {
         match packet {
             Some(mut packet) => {
+                if packet.data().len() > 0 {
                     if let Some(ref editor) = self.audio_filter {
                         editor.modify_stream(&mut packet.data_mut())
                     };
@@ -387,6 +388,7 @@ impl PlayerInternal {
                         self.stop_sink();
                     }
                 }
+            }
 
             None => {
                 self.stop_sink();