rocketchat: File upload chunks may exist without their metadata.

This is likely an error somewhere in rocketchat's MongoDB "eventual
consistency," but there is no problem with skipping the chunks at this
step.

In the one case where this was observed so far, the upload-id was not
referenced in any message -- if it is referenced and has chunks, but
has no metadata, we will fail later, at that reference.

zerver/data_import/rocketchat.py

@@ -765,6 +765,13 @@ def map_upload_id_to_upload_data(
         upload_id_to_upload_data_map[upload["_id"]] = {**upload, "chunk": []}
 
     for chunk in upload_data["chunk"]:
+        if chunk["files_id"] not in upload_id_to_upload_data_map:  # nocoverage
+            logging.info("Skipping chunk %s without metadata", chunk["files_id"])
+            # It's unclear why this apparent data corruption in the
+            # Rocket.Chat database is possible, but empirically, some
+            # chunks don't have any associated metadata.
+            continue
+
         upload_id_to_upload_data_map[chunk["files_id"]]["chunk"].append(chunk["data"])
 
     return upload_id_to_upload_data_map