Fixed docblock in UserInterface::getSalt()

This method can return null, too.
See the line above:

> "This can return null if the password was not encoded using a salt."
Merge branch '2.2' into 2.3

* 2.2:
  Fixed docblock in UserInterface::getSalt()
  [Process] Fix #8970 : read output once the process is finished, enable pipe tests on Windows
  [DoctrineBridge] Improved test coverage of EntityChoiceList
  [Form] Improved test coverage of ChoiceList classes
  [Form] Fixed expanded choice field to be marked invalid when unknown choices are submitted
  [Form] Fixed ChoiceList::get*By*() methods to preserve order and array keys
  [Form] Removed usage of the ChoiceList::getIndicesFor*() methods where they don't offer any performance benefit
  [HttpKernel] made code more reliable

Conflicts:
	src/Symfony/Bridge/Doctrine/Tests/Form/ChoiceList/EntityChoiceListTest.php
	src/Symfony/Component/Form/Extension/Core/ChoiceList/ChoiceListInterface.php
	src/Symfony/Component/Form/Extension/Core/EventListener/FixRadioInputListener.php
	src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php
	src/Symfony/Component/Form/Form.php
	src/Symfony/Component/Form/Tests/Extension/Core/Type/ChoiceTypeTest.php
	src/Symfony/Component/Process/Process.php
	src/Symfony/Component/Process/Tests/AbstractProcessTest.php