Merge pull request #1 from andrew-demb/arr-key-exists

Use `array_key_exists` instead `isset` (as null-values is not expected)

Author: mmasiukevich

src/Symfony/Extensions/PropertyNormalizerWrapper.php

@@ -49,7 +49,7 @@ protected function extractAttributes(object $object, string $format = null, arra
     {
         $class = \get_class($object);
 
-        if (false === isset($this->localStorage[$class]))
+        if (false === \array_key_exists($class, $this->localStorage))
         {
             $this->localStorage[$class] = parent::extractAttributes($object, $format, $context);
         }