Fixed the phpdoc of the VoterInterface
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