Fix NPE in HLS playback of non-muxed streams

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149749504

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

@@ -33,6 +33,7 @@ import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.IdentityHashMap;
 import java.util.List;
 
@@ -331,7 +332,7 @@ public final class HlsMediaPeriod implements MediaPeriod, HlsSampleStreamWrapper
     // Build audio stream wrappers.
     for (int i = 0; i < audioRenditions.size(); i++) {
       sampleStreamWrapper = buildSampleStreamWrapper(C.TRACK_TYPE_AUDIO,
-          new HlsUrl[] {audioRenditions.get(i)}, null, null);
+          new HlsUrl[] {audioRenditions.get(i)}, null, Collections.<Format>emptyList());
       sampleStreamWrappers[currentWrapperIndex++] = sampleStreamWrapper;
       sampleStreamWrapper.continuePreparing();
     }
@@ -340,7 +341,7 @@ public final class HlsMediaPeriod implements MediaPeriod, HlsSampleStreamWrapper
     for (int i = 0; i < subtitleRenditions.size(); i++) {
       HlsUrl url = subtitleRenditions.get(i);
       sampleStreamWrapper = buildSampleStreamWrapper(C.TRACK_TYPE_TEXT, new HlsUrl[] {url}, null,
-          null);
+          Collections.<Format>emptyList());
       sampleStreamWrapper.prepareSingleTrack(url.format);
       sampleStreamWrappers[currentWrapperIndex++] = sampleStreamWrapper;
     }