Remove E_NOTICE about undefined variable - refs BT#22195

Author: AngelFQC

main/admin/add_users_to_usergroup.php

@@ -203,8 +203,8 @@ function showLastTenUsers() {
 $first_letter_user = '';
 
 if ((isset($_POST['form_sent']) && $_POST['form_sent']) || isset($_REQUEST['firstLetterUser'])) {
-    $form_sent = $_POST['form_sent'];
-    $elements_posted = $_POST['elements_in_name'] ?? null;
+    $form_sent = $_POST['form_sent'] ?? 0;
+    $elements_posted = $_POST['elements_in_name'] ?? [];
     $first_letter_user = Security::remove_XSS($_REQUEST['firstLetterUser']);
 
     if (!is_array($elements_posted)) {