Several improvements

- Rename Guild#updateChannelPositions -> setChannelPositions
- Allow Guild#setChannelPositions to take ChannelResolvables
- Prioritise ClientDataResolver#resolveChannel's string case
- Minor cleanup

src/client/voice/player/AudioPlayer.js

@@ -55,7 +55,7 @@ class AudioPlayer extends EventEmitter {
    * @type {?StreamDispatcher}
    */
   get currentDispatcher() {
-    return (this.streams.last() || {}).dispatcher;
+    return this.streams.size > 0 ? this.streams.last().dispatcher || null : null;
   }
 
   destroy() {