if string is empty it should not use hash algorith

In the di https://github.com/auraphp/Aura.Auth/blob/develop-2/config/Common.php#L98

```php
$di->params['Aura\Auth\Verifier\PasswordVerifier'] = array(
            'algo' => '',
            'opts' => array()
        );
```

Which is empty, and was still calling the hash.