Fix PHP 8 warning in AjaxListener

[fritzmg]

Fixes the following warning:

ErrorException:
Warning: Undefined array key "load_callback"

  at vendor\codefog\contao-news_categories\src\EventListener\AjaxListener.php:119
  at Codefog\NewsCategoriesBundle\EventListener\AjaxListener->reloadNewsCategoriesWidget(object(DC_Table))
     (vendor\codefog\contao-news_categories\src\EventListener\AjaxListener.php:56)
  at Codefog\NewsCategoriesBundle\EventListener\AjaxListener->onExecutePostActions('reloadNewsCategoriesWidget', object(DC_Table))
     (vendor\contao\core-bundle\src\Resources\contao\classes\Ajax.php:527)
  at Contao\Ajax->executePostActionsHook(object(DC_Table))
     (vendor\contao\core-bundle\src\Resources\contao\classes\Ajax.php:509)
  at Contao\Ajax->executePostActions(object(DC_Table))
     (vendor\contao\core-bundle\src\Resources\contao\classes\Backend.php:430)
  at Contao\Backend->getBackendModule('themes', null)
     (vendor\contao\core-bundle\src\Resources\contao\controllers\BackendMain.php:168)
  at Contao\BackendMain->run()
     (vendor\contao\core-bundle\src\Controller\BackendController.php:49)
  at Contao\CoreBundle\Controller\BackendController->mainAction()
     (vendor\symfony\http-kernel\HttpKernel.php:163)
  at Symfony\Component\HttpKernel\HttpKernel->handleRaw(object(Request), 1)
     (vendor\symfony\http-kernel\HttpKernel.php:75)
  at Symfony\Component\HttpKernel\HttpKernel->handle(object(Request), 1, true)
     (vendor\symfony\http-kernel\Kernel.php:202)
  at Symfony\Component\HttpKernel\Kernel->handle(object(Request))
     (public\index.php:44)      

Otherwise you won't be able to select a reference category in the News categories list module for example (in the debug mode).

[qzminski]

Released as 3.4.23, thank you!