Add dedicated admin API for blocking a room

docs/admin_api/rooms.md

@@ -388,7 +388,8 @@ A response body like the following is returned:
 ```
 
 # Block Room API
-The Block Room admin API allows server admins to block and unblock rooms, and query to see if a given room is blocked.
+The Block Room admin API allows server admins to block and unblock rooms,
+and query to see if a given room is blocked.
 This API can be used to pre-emptively block a room, even if it's unknown to this
 homeserver. Users will be prevented from joining a blocked room.
 
@@ -460,8 +461,8 @@ The following parameters should be set in the URL:
 The following fields are possible in the JSON response body:
 
 - `block` - A boolean. `true` if the room is blocked, otherwise `false`
-- `user_id` - Optional. If the room is blocked (`block` is `true`) shows the user who has
-  add the room to blocking list. Otherwise it is not displayed.
+- `user_id` - A optional string. If the room is blocked (`block` is `true`) shows
+  the user who has add the room to blocking list. Otherwise it is not displayed.
 
 # Delete Room API
 

synapse/rest/admin/rooms.py

@@ -808,7 +808,9 @@ async def on_GET(
             )
 
         blocked_by = await self._store.room_is_blocked_by(room_id)
-        if blocked_by:
+        # Test `not None` if `user_id` is an empty string
+        # if someone add manually an entry in database
+        if blocked_by is not None:
             response = {"block": True, "user_id": blocked_by}
         else:
             response = {"block": False}

synapse/storage/databases/main/room.py

@@ -398,7 +398,11 @@ async def is_room_blocked(self, room_id: str) -> Optional[bool]:
         )
 
     async def room_is_blocked_by(self, room_id: str) -> Optional[str]:
-        """Function to retrieve user who has blocked the room"""
+        """
+        Function to retrieve user who has blocked the room.
+        user_id is non-nullable
+        It returns None if the room is not blocked.
+        """
         return await self.db_pool.simple_select_one_onecol(
             table="blocked_rooms",
             keyvalues={"room_id": room_id},