minor symfony#25846 [Serializer] Fix undefined class (chalasr)

This PR was merged into the 3.4 branch.

Discussion
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[Serializer] Fix undefined class

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

bad merge

Commits
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bc7b5d2 [Serializer] fix undefined class

Author: chalasr

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) {