Fix [scroll] with omitted x or y

As reported at https://forums.wesnoth.org/viewtopic.php?f=4&t=46168

data/lua/wml-tags.lua

@@ -739,7 +739,7 @@ function wml_actions.scroll(cfg)
 		end
 	end
 	if have_human or #sides == 0 then
-		wesnoth.scroll(cfg.x, cfg.y)
+		wesnoth.scroll(cfg.x or 0, cfg.y or 0)
 	end
 end