Resolve null pointer when empty playlist

src/main/java/net/Broken/audio/TrackScheduler.java

@@ -126,6 +126,7 @@ public class TrackScheduler extends AudioEventAdapter {
         // giving null to startTrack, which is a valid argument and will simply stop the player.
         UserAudioTrack track = queue.poll();
         this.currentPlayingTrack = track;
+        if(track != null)
             player.startTrack(track.getAudioTrack(), false);
     }