Add missing @Nullable for ExoPlayer.getPlayerError.

#minor-release

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

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java

@@ -982,6 +982,7 @@ public interface ExoPlayer extends Player {
    * {@link ExoPlaybackException}.
    * {@link ExoPlaybackException}.
    */
    */
   @Override
   @Override
+  @Nullable
   ExoPlaybackException getPlayerError();
   ExoPlaybackException getPlayerError();
 
 
   /**
   /**