Improve code readability and fix a minor bug.

Bug consist in the no execution of `$mvcAuthEvent->setIdentity(new Identity\GuestIdentity());` when it should, this happen when:

- Listener `$result` is a `Zend/Authentication/Result` instance but is not valid one (`$result->isValid()` return `false`).
- Listeners not set inside the MvcAuthEvent instance (or the Auth service) an identity (`$identity` then is `null`) neither a valid `Zend/Authentication/Result`.