Simplify some code with null coalesce operator

Author: javiereguiluz

Request.php

@@ -1371,7 +1371,7 @@ public function getRequestFormat($default = 'html')
             $this->format = $this->attributes->get('_format');
         }
 
-        return null === $this->format ? $default : $this->format;
+        return $this->format ?? $default;
     }
 
     /**

RequestStack.php

@@ -94,10 +94,6 @@ public function getParentRequest()
     {
         $pos = \count($this->requests) - 2;
 
-        if (!isset($this->requests[$pos])) {
-            return null;
-        }
-
-        return $this->requests[$pos];
+        return $this->requests[$pos] ?? null;
     }
 }

ResponseHeaderBag.php

@@ -176,7 +176,7 @@ public function hasCacheControlDirective($key)
      */
     public function getCacheControlDirective($key)
     {
-        return \array_key_exists($key, $this->computedCacheControl) ? $this->computedCacheControl[$key] : null;
+        return $this->computedCacheControl[$key] ?? null;
     }
 
     public function setCookie(Cookie $cookie)

Session/Storage/MetadataBag.php

@@ -163,6 +163,6 @@ private function stampCreated(int $lifetime = null): void
     {
         $timeStamp = time();
         $this->meta[self::CREATED] = $this->meta[self::UPDATED] = $this->lastUsed = $timeStamp;
-        $this->meta[self::LIFETIME] = (null === $lifetime) ? ini_get('session.cookie_lifetime') : $lifetime;
+        $this->meta[self::LIFETIME] = $lifetime ?? ini_get('session.cookie_lifetime');
     }
 }