[HttpFoundation] don't check twice if it's null

src/Symfony/Components/HttpFoundation/HeaderBag.php

@@ -207,9 +207,7 @@ class HeaderBag
                     throw new \InvalidArgumentException(sprintf('The "expires" cookie parameter is not valid.', $expires));
                 }
             }
-        }
 
-        if (null !== $expires) {
             $cookie .= '; expires='.substr(\DateTime::createFromFormat('U', $expires, new \DateTimeZone('UTC'))->format('D, d-M-Y H:i:s T'), 0, -5);
         }