Add a comment explaining the null check in DefaultMediaItemConverter

Follow-up to https://github.com/google/ExoPlayer/commit/42b8fe2681758b60f2d1e6df79d0727ce4a953d4

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()));
   }