Optimize list appends O(n^2)->O(n)

src/utility.c

@@ -116,7 +116,7 @@ struct ucl_object *ucl_list_append(struct ucl_object *list, struct ucl_object *o
 
     if (list->cell.car == NULL) {
         list->cell.car = obj;
-        return NULL;
+        return list;
     }
 
     while (iter->cell.cdr != NULL) {