Fix: Type error. ilObjSurvey::getAnonymousIdByCode() (#7410)

TypeError thrown with message "Return value of ilObjSurvey::getAnonymousIdByCode() must be of the type int, null returned"

Stacktrace:
#9 TypeError in /var/www/html/Modules/Survey/classes/class.ilObjSurvey.php:4702
#8 ilObjSurvey:getAnonymousIdByCode in /var/www/html/Modules/Survey/Execution/class.ilSurveyExecutionGUI.php:175
#7 ilSurveyExecutionGUI:checkAuth in /var/www/html/Modules/Survey/Execution/class.ilSurveyExecutionGUI.php:354
#6 ilSurveyExecutionGUI:outSurveyPage in /var/www/html/Modules/Survey/Execution/class.ilSurveyExecutionGUI.php:280
#5 ilSurveyExecutionGUI:redirectQuestion in /var/www/html/Modules/Survey/Execution/class.ilSurveyExecutionGUI.php:140
#4 ilSurveyExecutionGUI:executeCommand in /var/www/html/Services/UICore/classes/class.ilCtrl.php:199
#3 ilCtrl:forwardCommand in /var/www/html/Modules/Survey/classes/class.ilObjSurveyGUI.php:206
#2 ilObjSurveyGUI:executeCommand in /var/www/html/Services/UICore/classes/class.ilCtrl.php:199
#1 ilCtrl:forwardCommand in /var/www/html/Services/UICore/classes/class.ilCtrl.php:174
#0 ilCtrl:callBaseClass in /var/www/html/ilias.php:24

components/ILIAS/Survey/classes/class.ilObjSurvey.php

@@ -4684,13 +4684,13 @@ public function getAppraiseesToRate(
      */
     public function getAnonymousIdByCode(
         string $a_code
-    ): int {
+    ): ?int {
         $ilDB = $this->db;
         $set = $ilDB->query("SELECT anonymous_id FROM svy_anonymous" .
                 " WHERE survey_fi = " . $ilDB->quote($this->getSurveyId(), "integer") .
                 " AND survey_key = " . $ilDB->quote($a_code, "text"));
         $res = $ilDB->fetchAssoc($set);
-        return $res["anonymous_id"];
+        return $res["anonymous_id"] ?? null;
     }
 
     /**