We're not playing an ad if the timeline is empty.

Issue: #3334

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172447125

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

@@ -386,17 +386,17 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public boolean isPlayingAd() {
-    return pendingSeekAcks == 0 && playbackInfo.periodId.isAd();
+    return !timeline.isEmpty() && pendingSeekAcks == 0 && playbackInfo.periodId.isAd();
   }
 
   @Override
   public int getCurrentAdGroupIndex() {
-    return pendingSeekAcks == 0 ? playbackInfo.periodId.adGroupIndex : C.INDEX_UNSET;
+    return isPlayingAd() ? playbackInfo.periodId.adGroupIndex : C.INDEX_UNSET;
   }
 
   @Override
   public int getCurrentAdIndexInAdGroup() {
-    return pendingSeekAcks == 0 ? playbackInfo.periodId.adIndexInAdGroup : C.INDEX_UNSET;
+    return isPlayingAd() ? playbackInfo.periodId.adIndexInAdGroup : C.INDEX_UNSET;
   }
 
   @Override