unified return null usages
[Security] fix DBAL connection typehint
minor #10717 unified return null usages (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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unified return null usages

| Q             | A
| ------------- | ---
| License       | MIT

This PR unifies the way we return `null` from a function or method:

 * always use `return;` instead of `return null;` (the current code base uses both);
 * never use `return;` at the end of a function/method.

Commits
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d1d569b unified return null usages
Merge branch '2.3' into 2.4

* 2.3:
  Textarea value should default to empty string instead of null.
  Fix doc blocks
  [Process] Fix #10681, process are failing on Windows Server 2003
  [Security] fix DBAL connection typehint
  [HttpFoundation] status 201 is allowed to have a body
  unified return null usages

Conflicts:
	src/Symfony/Component/Console/Helper/DescriptorHelper.php
	src/Symfony/Component/DependencyInjection/ContainerBuilder.php
	src/Symfony/Component/Finder/Tests/Iterator/RecursiveDirectoryIteratorTest.php
	src/Symfony/Component/Form/Extension/Core/DataTransformer/IntegerToLocalizedStringTransformer.php
	src/Symfony/Component/HttpKernel/Fragment/InlineFragmentRenderer.php