Fix HLS media playlist only playback

This was broken by []

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182206548

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

@@ -932,6 +932,9 @@ import java.util.Arrays;
     int sampleTrackType = MimeTypes.getTrackType(sampleFormat.sampleMimeType);
     String codecs = Util.getCodecsOfType(playlistFormat.codecs, sampleTrackType);
     String mimeType = MimeTypes.getMediaMimeType(codecs);
+    if (mimeType == null) {
+      mimeType = sampleFormat.sampleMimeType;
+    }
     return sampleFormat.copyWithContainerInfo(
         playlistFormat.id,
         mimeType,