Replace timeline reference equality checks in FakeMediaSource

#minor-release

PiperOrigin-RevId: 363141851

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java

@@ -204,14 +204,14 @@ public class FakeMediaSource extends BaseMediaSource {
   @Override
   @Nullable
   public Timeline getInitialTimeline() {
-    return timeline == null || timeline == Timeline.EMPTY || timeline.getWindowCount() == 1
+    return timeline == null || timeline.isEmpty() || timeline.getWindowCount() == 1
         ? null
         : new InitialTimeline(timeline);
   }
 
   @Override
   public boolean isSingleWindow() {
-    return timeline == null || timeline == Timeline.EMPTY || timeline.getWindowCount() == 1;
+    return timeline == null || timeline.isEmpty() || timeline.getWindowCount() == 1;
   }
 
   @Override