minor #11758 Fixed the phpdoc of the VoterInterface (stof)

This PR was merged into the 2.3 branch.

Discussion
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Fixed the phpdoc of the VoterInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | n/a

``null`` is an accepted value for voters (it is passed when you are not voting on a specific object, for instance for roles.

Commits
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b591404 Fixed the phpdoc of the VoterInterface

src/Symfony/Component/Security/Core/Authorization/Voter/VoterInterface.php

@@ -49,7 +49,7 @@ public function supportsClass($class);
      * ACCESS_GRANTED, ACCESS_DENIED, or ACCESS_ABSTAIN.
      *
      * @param TokenInterface $token      A TokenInterface instance
-     * @param object         $object     The object to secure
+     * @param object|null    $object     The object to secure
      * @param array          $attributes An array of attributes associated with the method being invoked
      *
      * @return int     either ACCESS_GRANTED, ACCESS_ABSTAIN, or ACCESS_DENIED