Allow returning null from NormalizerInterface::normalize

[teohhanhui]

Q	A
Branch?	3.4
Bug fix?	yes?
New feature?	no
Deprecations?	no
Tickets	N/A
License	MIT
Doc PR	N/A

Looking at the code, it seems that a normalizer might be called with a null value for $data, and thus it's only sensible that it be allowed to return null too:

symfony/src/Symfony/Component/Serializer/Serializer.php

Lines 141 to 148 in 7064ff3

	// If a normalizer supports the given data, use it
	if ($normalizer = $this->getNormalizer($data, $format, $context)) {
	return $normalizer->normalize($data, $format, $context);
	}
	if (null === $data || is_scalar($data)) {
	return $data;
	}

Updating the phpdoc to match.

[nicolas-grekas]

Thank you @teohhanhui