User: Fix self registration crashes if text fields are not required

See: https://mantis.ilias.de/view.php?id=42869

components/ILIAS/User/classes/Profile/class.ilUserProfile.php

@@ -377,6 +377,7 @@ private function getTextInput(
         ?ilObjUser $user
     ): ilFormPropertyGUI {
         $text_input = new ilTextInputGUI($this->lng->txt($lang_var), 'usr_' . $field_id);
+        $text_input->setValue('');
         if ($user !== null) {
             $text_input->setValue($user->$method() ?? '');
         }