Fix HlsMediaPlaylist download using HlsDownloadHelper

Issue: #4396

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201324467

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/offline/HlsDownloadHelper.java

@@ -73,6 +73,7 @@ public final class HlsDownloadHelper extends DownloadHelper {
   public TrackGroupArray getTrackGroups(int periodIndex) {
     Assertions.checkNotNull(playlist);
     if (playlist instanceof HlsMediaPlaylist) {
+      renditionGroups = new int[0];
       return TrackGroupArray.EMPTY;
     }
     // TODO: Generate track groups as in playback. Reverse the mapping in getDownloadAction.