do not use unavailable fn

Author: zth

compiler/ext/ext_list.ml

@@ -774,3 +774,7 @@ let filter lst p =
     | x :: l -> if p x then find (x :: accu) l ~p else find accu l ~p
   in
   find [] lst ~p
+
+let is_empty = function
+  | [] -> true
+  | _ :: _ -> false

compiler/ext/ext_list.mli

@@ -231,3 +231,5 @@ val filter : 'a list -> ('a -> bool) -> 'a list
 
 val array_list_filter_map :
   'a array -> 'b list -> ('a -> 'b -> 'c option) -> 'c list
+
+val is_empty : 'a list -> bool

tools/src/tools.ml

@@ -1625,7 +1625,7 @@ module Migrate = struct
                   MapperUtils.get_template_args_to_insert mapper template_args
                     []
                 in
-                if List.is_empty template_args_to_insert then
+                if Ext_list.is_empty template_args_to_insert then
                   {
                     exp with
                     pexp_desc =