[5.4] Fix handling of responses from autoupdate server (joomla#45673)

Fix response handling for undefined array

---------

Co-authored-by: @Ruud68

Author: SniperSister

administrator/components/com_joomlaupdate/src/Model/UpdateModel.php

@@ -614,8 +614,8 @@ public function changeAutoUpdateRegistration(AutoupdateRegisterState $targetStat
             Factory::getApplication()->enqueueMessage(
                 Text::sprintf(
                     'COM_JOOMLAUPDATE_AUTOUPDATE_REGISTER_ERROR',
-                    $result['message'] ?: '',
-                    $result['status'] ?: ''
+                    $result['message'] ?? '',
+                    $result['status'] ?? ''
                 ),
                 'error'
             );