patch 9.0.0345: error message for list argument could be clearer

Problem:    Error message for list argument could be clearer.
Solution:   Include the argument number. (Yegappan Lakshmanan, closes #11027)
diff --git a/src/mbyte.c b/src/mbyte.c
index 73065c7..54f7fa4 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -5535,14 +5535,9 @@
     size_t	    cw_table_size_save;
     char	    *error = NULL;
 
-    if (in_vim9script() && check_for_list_arg(argvars, 0) == FAIL)
+    if (check_for_nonnull_list_arg(argvars, 0) == FAIL)
 	return;
 
-    if (argvars[0].v_type != VAR_LIST || argvars[0].vval.v_list == NULL)
-    {
-	emsg(_(e_list_required));
-	return;
-    }
     l = argvars[0].vval.v_list;
     if (l->lv_len == 0)
     {