[BUGFIX] Initialize strictly typed property setCookie with null

Releases: master
Resolves: #93069
Relates: #93011
Change-Id: I80c970009694462d48ec65f96eb9b0f29cacf18c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67108
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Classes/Authentication/AbstractUserAuthentication.php

@@ -212,7 +212,7 @@ abstract class AbstractUserAuthentication implements LoggerAwareInterface
      *
      * @var Cookie|null
      */
-    protected ?Cookie $setCookie;
+    protected ?Cookie $setCookie = null;
 
     /**
      * Initialize some important variables
@@ -285,7 +285,7 @@ public function start()
      */
     public function appendCookieToResponse(ResponseInterface $response): ResponseInterface
     {
-        if (isset($this->setCookie)) {
+        if ($this->setCookie !== null) {
             $response = $response->withAddedHeader('Set-Cookie', $this->setCookie->__toString());
         }
         return $response;