Send empty timeline for empty DynamicConcatenatingMediaSource immediately.

Without child sources, the timeline can't possibly updated again within the
current looper message. Thus, we can send the empty timeline immediately.

This fixes a recently introduced flakiness in the test.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187295700

library/core/src/main/java/com/google/android/exoplayer2/source/DynamicConcatenatingMediaSource.java

@@ -322,9 +322,13 @@ public final class DynamicConcatenatingMediaSource extends CompositeMediaSource<
   public synchronized void prepareSourceInternal(ExoPlayer player, boolean isTopLevelSource) {
     super.prepareSourceInternal(player, isTopLevelSource);
     this.player = player;
-    shuffleOrder = shuffleOrder.cloneAndInsert(0, mediaSourcesPublic.size());
-    addMediaSourcesInternal(0, mediaSourcesPublic);
-    scheduleListenerNotification(/* actionOnCompletion= */ null);
+    if (mediaSourcesPublic.isEmpty()) {
+      notifyListener();
+    } else {
+      shuffleOrder = shuffleOrder.cloneAndInsert(0, mediaSourcesPublic.size());
+      addMediaSourcesInternal(0, mediaSourcesPublic);
+      scheduleListenerNotification(/* actionOnCompletion= */ null);
+    }
   }
 
   @Override