When getting the session's id, check if the session is not closed

This introduced a regression from #9246, with an incomplete fix ;
As the `started` flag on the NativeSessionStorage was not `true`
anymore when saving the session, the session id was always empty
when saving it, and thus when sending the `PHPSESSID` cookie
Merge branch '2.2' into 2.3

* 2.2:
  Teardown used wrong property
  Modified guessDefaultEscapingStrategy to not escape txt templates
  Fix DateType for 32bits computers.
  Fixed the registration of validation.xml file when the form is disabled
  When getting the session's id, check if the session is not closed
Merge branch '2.3' into 2.4

* 2.3:
  [Debug] fixed unit tests
  Avoid notice from being *eaten* by fatal error.
  Teardown used wrong property
  Modified guessDefaultEscapingStrategy to not escape txt templates
  Fix DateType for 32bits computers.
  Fixed the registration of validation.xml file when the form is disabled
  Fixes #9633, Removed dependency to Symfony\Bundle\FrameworkBundle\Tests\TestCase
  [Validator] Replaced inexistent interface.
  When getting the session's id, check if the session is not closed
  Adjusting CacheClear Warmup method to namespaced kernels

Conflicts:
	src/Symfony/Bundle/FrameworkBundle/Command/CacheClearCommand.php