Fix crash when building the play queue audio track menu if the player is null

As the player can be null in some cases, we have to make sure that the player
is not null, by using Optionals on the player itself instead of its methods
returning Optionals.

If the player is null, the play queue audio track menu will now be hidden.

app/src/main/java/org/schabi/newpipe/player/PlayQueueActivity.java

@@ -619,11 +619,13 @@ public final class PlayQueueActivity extends AppCompatActivity
 
         final MenuItem audioTrackSelector = menu.findItem(R.id.action_audio_track);
         final List<AudioStream> availableStreams =
-                Optional.ofNullable(player.getCurrentMetadata())
+                Optional.ofNullable(player)
+                        .map(Player::getCurrentMetadata)
                         .flatMap(MediaItemTag::getMaybeAudioTrack)
                         .map(MediaItemTag.AudioTrack::getAudioStreams)
                         .orElse(null);
-        final Optional<AudioStream> selectedAudioStream = player.getSelectedAudioStream();
+        final Optional<AudioStream> selectedAudioStream = Optional.ofNullable(player)
+                .flatMap(Player::getSelectedAudioStream);
 
         if (availableStreams == null || availableStreams.size() < 2
                 || selectedAudioStream.isEmpty()) {