Don't query empty timeline for next/previous windows

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173668909

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

@@ -331,14 +331,14 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public int getNextWindowIndex() {
-    return timeline.getNextWindowIndex(getCurrentWindowIndex(), getRepeatMode(),
-        getShuffleModeEnabled());
+    return timeline.isEmpty() ? C.INDEX_UNSET
+        : timeline.getNextWindowIndex(getCurrentWindowIndex(), repeatMode, shuffleModeEnabled);
   }
 
   @Override
   public int getPreviousWindowIndex() {
-    return timeline.getPreviousWindowIndex(getCurrentWindowIndex(), getRepeatMode(),
-        getShuffleModeEnabled());
+    return timeline.isEmpty() ? C.INDEX_UNSET
+        : timeline.getPreviousWindowIndex(getCurrentWindowIndex(), repeatMode, shuffleModeEnabled);
   }
 
   @Override