minor #29408 [Security] getTargetPath of TargetPathTrait must return string or null (gmponos)

This PR was squashed before being merged into the 3.4 branch (closes #29408).

Discussion
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[Security] getTargetPath of TargetPathTrait must return string or null

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes (possible bug)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Since the return type is string the default return value must be also string.

Commits
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8d4b787dd9 [Security] getTargetPath of TargetPathTrait must return string or null

Author: fabpot

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)
     {