Fix fatal error in ExtractorQueue::processItem()

There is a slight bug in the changes we made in #3601364: Queued files in key-value store are not properly deleted that can lead to a fatal error:

      $fallback_collection = $this->keyValue->get(FilesExtractor::FALLBACK_QUEUE_KV);
      $queued_files = $fallback_collection->get($data->fid);
      unset($queued_files[$data->entity_type][$data->entity_id]);
      $queued_files = array_filter($queued_files);

If the fallback collection for the file is empty, then $queued_files will be NULL so array_filter() will produce a TypeError.

Sorry for being sloppy there!