Note that new_room_id may be null

docs/admin_api/rooms.md

@@ -478,7 +478,8 @@ The following fields are returned in the JSON response body:
 * `failed_to_kick_users` - An array of users (`user_id`) that that were not kicked.
 * `local_aliases` - An array of strings representing the local aliases that were migrated from
                     the old room to the new.
-* `new_room_id` - A string representing the room ID of the new room.
+* `new_room_id` - A string representing the room ID of the new room, or `null` if
+                  no such room was created.
 
 
 ## Undoing room deletions

synapse/handlers/room.py

@@ -1344,7 +1344,9 @@ async def shutdown_room(
             local_aliases:
                 An array of strings representing the local aliases that were
                 migrated from the old room to the new.
-            new_room_id: A string representing the room ID of the new room.
+            new_room_id:
+                A string representing the room ID of the new room, or None if
+                no such room was created.
         """
 
         if not new_room_name: