[REF] Refactor redundant `isset()`s to use null coalescing operator (??).

CRM/Core/BAO/CMSUser.php

CRM/Event/Form/ManageEvent.php

CRM/Event/Form/ManageEvent/TabHeader.php

CRM/Utils/Mail.php

CRM/Utils/SameSite.php

CRM/Utils/System/WordPress.php

Civi/Core/CiviEventDispatcher.php

Civi/Pipe/PublicMethods.php

ext/afform/mock/tests/phpunit/api/v4/Afform/AfformTest.php

ext/flexmailer/src/FlexMailerTask.php

ext/flexmailer/src/Listener/DefaultComposer.php