Add missing @Nullable for ExoPlayer.getPlayerError.

#minor-release

Issue: google/ExoPlayer#9660
PiperOrigin-RevId: 408323173

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/ExoPlayer.java

@@ -1018,6 +1018,7 @@ public interface ExoPlayer extends Player {
    * {@link ExoPlaybackException}.
    */
   @Override
+  @Nullable
   ExoPlaybackException getPlayerError();
 
   /**