properly handle null data when denormalizing

If null is passed to denormalize(), no property values can be set on
the denormalized object. Additionally, this fixes passing values to
the denormalized object's constructor if the incoming data is an object.
fixed CS
bug #11428 [Serializer] properly handle null data when denormalizing (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Serializer] properly handle null data when denormalizing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10794
| License       | MIT
| Doc PR        |

Commits
-------

123fc62 properly handle null data when denormalizing
Merge branch '2.3' into 2.4

* 2.3: (22 commits)
  Fix incorrect romanian plural translations
  fix axes handling in Crawler::filterXPath()
  fix some docblocks
  Fixed self-reference in 'service_container' service breaks garbage collection (and clone).
  [Process] Fix tests when pcntl is not available.
  [DependencyInjection] Roll back changes made to generated files.
  [Console] Roll back changes made to fixture files.
  [Validator] Added more detailed inline documentation
  [Validator] Removed information from the violation output if the value is an array, object or resource
  partially reverted previous commit
  fixed CS
  properly handle null data when denormalizing
  [Validator] Renamed valueToString() to formatValue(); added missing formatValue() calls
  [Validator] Fixed CS
  [Validator] Fixed date-to-string conversion tests to match ICU 51
  [Validator] Added "{{ value }}" parameters where they were missing
  [Validator] Simplified and explained the LuhnValidator
  [Validator] Simplified IssnValidator
  [Validator] Fixed and simplified IsbnValidator
  [Validator] Simplified IBAN validation algorithm
  ...

Conflicts:
	src/Symfony/Component/Console/Helper/DescriptorHelper.php
	src/Symfony/Component/DependencyInjection/Container.php
	src/Symfony/Component/DomCrawler/Tests/CrawlerTest.php
	src/Symfony/Component/HttpFoundation/File/UploadedFile.php
	src/Symfony/Component/HttpKernel/Fragment/FragmentHandler.php
	src/Symfony/Component/Validator/Constraints/AbstractComparisonValidator.php
	src/Symfony/Component/Validator/Constraints/CollectionValidator.php
	src/Symfony/Component/Validator/Tests/Constraints/EqualToValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/GreaterThanValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/IdenticalToValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/LessThanOrEqualValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/LessThanValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/NotEqualToValidatorTest.php
	src/Symfony/Component/Validator/Tests/Constraints/NotIdenticalToValidatorTest.php