Issue # 3514830: Added validation in the strcasecmp() call to check if...

Issue # 3514830: Added validation in the strcasecmp() call to check if ->getResponseHeader('vary') exists and is not null

Closes #3514830

core/modules/system/tests/src/Functional/Routing/RouterTest.php

@@ -47,7 +47,7 @@ public function testFinishResponseSubscriber(): void {
     $this->assertSession()->responseHeaderEquals('Content-language', 'en');
     $this->assertSession()->responseHeaderEquals('X-Content-Type-Options', 'nosniff');
     $this->assertSession()->responseHeaderEquals('X-Frame-Options', 'SAMEORIGIN');
-    if (strcasecmp($session->getResponseHeader('vary'), 'accept-encoding') !== 0) {
+    if (strcasecmp($session->getResponseHeader('vary') ?? "", 'accept-encoding') !== 0) {
       $this->assertSession()->responseHeaderDoesNotExist('Vary');
     }