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.