Update src/JMS/Serializer/Construction/DoctrineObjectConstructor.php

Change isset() to array_key_exists() as Doctrine allows composite PK with nulls.

src/JMS/Serializer/Construction/DoctrineObjectConstructor.php

@@ -74,7 +74,7 @@ class DoctrineObjectConstructor implements ObjectConstructorInterface
         $identifierList = array();
 
         foreach ($classMetadata->getIdentifierFieldNames() as $name) {
-            if ( ! isset($data[$name])) {
+            if ( ! array_key_exists($name, $data)) {
                 return $this->fallbackConstructor->construct($visitor, $metadata, $data, $type);
             }