Fix PlaybackControlView's repeat mode button update when player is null

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162824522

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlaybackControlView.java

@@ -660,6 +660,11 @@ public class PlaybackControlView extends FrameLayout {
       repeatToggleButton.setVisibility(View.GONE);
       return;
     }
+    if (player == null) {
+      setButtonEnabled(false, repeatToggleButton);
+      return;
+    }
+    setButtonEnabled(true, repeatToggleButton);
     switch (player.getRepeatMode()) {
       case Player.REPEAT_MODE_OFF:
         repeatToggleButton.setImageDrawable(repeatOffButtonDrawable);