Disable riot-web's welcome bot

I've not found this welcome bot to work at all in my previous attempts.
It would simply not reply, even though federation works.

It seems like this is also a potential privacy issue, as per
https://gist.github.com/maxidorius/5736fd09c9194b7a6dc03b6b8d7220d0

group_vars/matrix_servers

@@ -438,6 +438,8 @@ matrix_riot_web_enable_presence_by_hs_url: |
     else {matrix_riot_web_default_hs_url: false}
   }}
 
+matrix_riot_web_welcome_user_id: ~
+
 ######################################################################
 #
 # /matrix-riot-web

roles/matrix-riot-web/templates/config.json.j2

@@ -14,7 +14,7 @@
 	"roomDirectory": {
 		"servers": {{ matrix_riot_web_roomdir_servers|to_json }}
 	},
-	"welcomeUserId": {{ matrix_riot_web_welcome_user_id|string|to_json }},
+	"welcomeUserId": {{ matrix_riot_web_welcome_user_id|to_json }},
 	{% if matrix_riot_web_enable_presence_by_hs_url is not none %}
 		"enable_presence_by_hs_url": {{ matrix_riot_web_enable_presence_by_hs_url|to_json }},
 	{% endif %}