Fix FlacStreamMetadata compile failure

Comment concatenation was accidentally not added previously, which
results in a testing failure.

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/FlacStreamMetadata.java

@@ -143,8 +143,7 @@ public final class FlacStreamMetadata {
         bitsPerSample,
         bitsPerSample,
         totalSamples,
         totalSamples,
         /* seekTable= */ null,
         /* seekTable= */ null,
-        new Metadata(pictureFrames)
-          .copyWithAppendedEntriesFrom(VorbisUtil.parseVorbisComments(vorbisComments)));
+        concatenateVorbisMetadata(vorbisComments, pictureFrames));
   }
   }
 
 
   private FlacStreamMetadata(
   private FlacStreamMetadata(
@@ -250,6 +249,22 @@ public final class FlacStreamMetadata {
     return metadata == null ? other : metadata.copyWithAppendedEntriesFrom(other);
     return metadata == null ? other : metadata.copyWithAppendedEntriesFrom(other);
   }
   }
 
 
+  /** Returns */
+  @Nullable
+  private static Metadata concatenateVorbisMetadata(
+      List<String> vorbisComments,
+      List<PictureFrame> pictureFrames
+  ) {
+    @Nullable Metadata parsedVorbisComments = VorbisUtil.parseVorbisComments(vorbisComments);
+
+    if (parsedVorbisComments == null && pictureFrames.isEmpty()) {
+      return null;
+    }
+
+    return new Metadata(pictureFrames)
+        .copyWithAppendedEntriesFrom(parsedVorbisComments);
+  }
+
   /** Returns a copy of {@code this} with the seek table replaced by the one given. */
   /** Returns a copy of {@code this} with the seek table replaced by the one given. */
   public FlacStreamMetadata copyWithSeekTable(@Nullable SeekTable seekTable) {
   public FlacStreamMetadata copyWithSeekTable(@Nullable SeekTable seekTable) {
     return new FlacStreamMetadata(
     return new FlacStreamMetadata(