Update RestController to handle entities with $id = 0

To make the `RestController` properly handle entities with identifier `$id = 0` this change needs to be made to the if-clause in the `getIdentifier` method.
This is related to issues https://github.com/zfcampus/zf-rest/issues/35 and https://github.com/zfcampus/zf-hal/issues/44.