Fix flaky testEmptyTimeline again.

Waiting for the timeline change didn't work correctly because the timeline was
already equal to Timeline.EMPTY (due to the masking). Now waiting explicitly
for the empty Timeline exposed by the source.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177749292

library/core/src/androidTest/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -54,7 +54,7 @@ public final class ExoPlayerTest extends TestCase {
    * error.
    * error.
    */
    */
   public void testPlayEmptyTimeline() throws Exception {
   public void testPlayEmptyTimeline() throws Exception {
-    Timeline timeline = Timeline.EMPTY;
+    Timeline timeline = new FakeTimeline(/* windowCount= */ 0);
     FakeRenderer renderer = new FakeRenderer();
     FakeRenderer renderer = new FakeRenderer();
     // TODO(b/69665207): Without waiting for the timeline update, this test is flaky as the timeline
     // TODO(b/69665207): Without waiting for the timeline update, this test is flaky as the timeline
     // update happens after the transition to STATE_ENDED and the test runner may already have been
     // update happens after the transition to STATE_ENDED and the test runner may already have been