Add a comment explaining the null check in DefaultMediaItemConverter

Follow-up to 42b8fe2681

PiperOrigin-RevId: 301352557

extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultMediaItemConverter.java

@@ -44,6 +44,7 @@ public final class DefaultMediaItemConverter implements MediaItemConverter {
 
   @Override
   public MediaItem toMediaItem(MediaQueueItem item) {
+    // `item` came from `toMediaQueueItem()` so the custom JSON data must be set.
     return getMediaItem(Assertions.checkNotNull(item.getMedia().getCustomData()));
   }