terraria service: fixup worldPath option type

Otherwise, using the defaults results in a type error.

nixos/modules/services/games/terraria.nix

···
       };
 
       worldPath = mkOption {
-        type        = types.path;
+        type        = types.nullOr types.path;
         default     = null;
         description = ''
           The path to the world file (<literal>.wld</literal>) which should be loaded.