Merge branch '3.4' into 4.1

* 3.4:
  [Debug] ignore underscore vs backslash namespaces in DebugClassLoader
  [TwigBridge][Form] Prevent multiple rendering of form collection prototypes
  [FrameworkBundle] fix describing routes with no controllers
  [DI] move RegisterServiceSubscribersPass before DecoratorServicePass
  Update ValidationListener.php
  [Yaml] ensures that the mb_internal_encoding is reset to its initial value
  [WebLink] Fixed documentation link
  [Security] getTargetPath of TargetPathTrait must return string or null
  [Hackday][Serializer] Deserialization ignores argument type hint from phpdoc for array in constructor argument
  [Security] defer log message in guard authenticator
  merge conflicts
  Fix HeaderBag::get phpdoc

Author: nicolas-grekas

Util/TargetPathTrait.php

@@ -38,7 +38,7 @@ private function saveTargetPath(SessionInterface $session, $providerKey, $uri)
      * @param SessionInterface $session
      * @param string           $providerKey The name of your firewall
      *
-     * @return string
+     * @return string|null
      */
     private function getTargetPath(SessionInterface $session, $providerKey)
     {