[EventDispatcher] don't count empty listeners

When event listeners for certain events are removed from the event
dispatcher, empty arrays are not being removed. Therefore, counting
on empty arrays leads to wrong results of the hasListeners() method.
Merge branch '2.3' into 2.4

* 2.3:
  Update validators.eu.xlf
  fixed CS
  remove unused imports
  Unify null comparisons
  [EventDispatcher] don't count empty listeners
  [Process] Fix unit tests in sigchild environment
  [Process] fix signal handling in wait()
  [BrowserKit] refactor code and fix unquoted regex
  Fixed server HTTP_HOST port uri conversion
  Bring code into standard
  [Process] Add test to verify fix for issue #11421
  [Process] Fixes issue #11421
  [DependencyInjection] Pass a Scope instance instead of a scope name.

Conflicts:
	src/Symfony/Component/EventDispatcher/Tests/EventDispatcherTest.php