[xml] do not wrap optional args type with <x path="Null">

src/codegen/genxml.ml

@@ -70,7 +70,7 @@ let tpath t =
 	real_path i.mt_path i.mt_meta
 
 let rec follow_param t =
-	match t with
+	match (follow_lazy t) with
 	| TMono r ->
 		(match r.tm_type with
 		| Some t -> follow_param t