prosody muc_size plugin, room get info error fix (Traceback[httpserver]: /usr/lib/prosody/util/async.lua:137: /prosody-plugins/mod_muc_size.lua:141: attempt to concatenate local 'subdomain' (a nil value)

resources/prosody-plugins/mod_muc_size.lua

     local room_address
         = jid.join(room_name, muc_domain_prefix.."."..domain_name);
 
-    if subdomain ~= "" then
+    if subdomain and subdomain ~= "" then
         room_address = "["..subdomain.."]"..room_address;
     end