[Serializer] fix undefined class

Author: Robin Chalas

src/Symfony/Component/Serializer/Normalizer/DateTimeNormalizer.php

@@ -84,7 +84,7 @@ public function denormalize($data, $class, $format = null, array $context = arra
         $timezone = $this->getTimezone($context);
 
         if ('' === $data || null === $data) {
-            throw new UnexpectedValueException('The data is either an empty string or null, you should pass a string that can be parsed with the passed format or a valid DateTime string.');
+            throw new NotNormalizableValueException('The data is either an empty string or null, you should pass a string that can be parsed with the passed format or a valid DateTime string.');
         }
 
         if (null !== $dateTimeFormat) {