minor #42165 Simplify some code with null coalesce operator (javiereguiluz)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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Simplify some code with null coalesce operator

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

For your consideration. There are many other possible usages of null coalesce operator, but the result is not very readable ... so this PR only contains the changes where the result is clearly better.

Commits
-------

17ad5b75fa Simplify some code with null coalesce operator

Author: derrabus

Firewall/RememberMeListener.php

@@ -58,7 +58,7 @@ public function __construct(TokenStorageInterface $tokenStorage, RememberMeServi
         }
 
         $this->catchExceptions = $catchExceptions;
-        $this->sessionStrategy = null === $sessionStrategy ? new SessionAuthenticationStrategy(SessionAuthenticationStrategy::MIGRATE) : $sessionStrategy;
+        $this->sessionStrategy = $sessionStrategy ?? new SessionAuthenticationStrategy(SessionAuthenticationStrategy::MIGRATE);
     }
 
     /**

Tests/Firewall/ExceptionListenerTest.php

@@ -99,7 +99,7 @@ public function testAccessDeniedExceptionFullFledgedAndWithoutAccessDeniedHandle
         $listener->onKernelException($event);
 
         $this->assertNull($event->getResponse());
-        $this->assertSame(null === $eventException ? $exception : $eventException, $event->getThrowable()->getPrevious());
+        $this->assertSame($eventException ?? $exception, $event->getThrowable()->getPrevious());
     }
 
     /**
@@ -122,7 +122,7 @@ public function testAccessDeniedExceptionFullFledgedAndWithoutAccessDeniedHandle
 
         $this->assertEquals('Unauthorized', $event->getResponse()->getContent());
         $this->assertEquals(401, $event->getResponse()->getStatusCode());
-        $this->assertSame(null === $eventException ? $exception : $eventException, $event->getThrowable()->getPrevious());
+        $this->assertSame($eventException ?? $exception, $event->getThrowable()->getPrevious());
     }
 
     /**
@@ -139,7 +139,7 @@ public function testAccessDeniedExceptionFullFledgedAndWithAccessDeniedHandlerAn
         $listener->onKernelException($event);
 
         $this->assertEquals('error', $event->getResponse()->getContent());
-        $this->assertSame(null === $eventException ? $exception : $eventException, $event->getThrowable()->getPrevious());
+        $this->assertSame($eventException ?? $exception, $event->getThrowable()->getPrevious());
     }
 
     /**
@@ -156,7 +156,7 @@ public function testAccessDeniedExceptionNotFullFledged(\Exception $exception, \
         $listener->onKernelException($event);
 
         $this->assertEquals('OK', $event->getResponse()->getContent());
-        $this->assertSame(null === $eventException ? $exception : $eventException, $event->getThrowable()->getPrevious());
+        $this->assertSame($eventException ?? $exception, $event->getThrowable()->getPrevious());
     }
 
     public function testLogoutException()