Fix Null audio driver

null/null.go

@@ -71,6 +71,11 @@ func (p Playable) Play(loops int) error {
 	return nil
 }
 
+// Playing?
+func (p Playable) Playing() bool {
+	return false
+}
+
 // Pause nothing
 func (p Playable) Pause() error {
 	return nil