Fix NullMetadata performance issue

Author: adrienbrault

src/Metadata/MetadataFactory.php

@@ -112,12 +112,12 @@ public function getMetadataForClass($className)
             }
 
             if (null !== $this->cache && !$this->debug) {
-                $this->cache->putClassMetadataInCache(new NullMetadata());
+                $this->cache->putClassMetadataInCache(new NullMetadata($class->getName()));
             }
         }
 
         if (null === $metadata) {
-            $metadata = new NullMetadata();
+            $metadata = new NullMetadata($className);
         }
 
         return $this->filterNullMetadata($this->loadedMetadata[$className] = $metadata);

src/Metadata/NullMetadata.php

@@ -9,18 +9,5 @@
  */
 class NullMetadata extends ClassMetadata
 {
-    public function __construct()
-    {
 
-    }
-
-    public function serialize()
-    {
-        return '';
-    }
-
-    public function unserialize($str)
-    {
-
-    }
 }