OAuth2 Mongo Adapter cannot be created successfully

According to this [thread](https://groups.google.com/a/zend.com/d/msgid/apigility-users/23703bf6-1ada-4085-8e24-a09aa66fc3d8%40zend.com?utm_medium=email&utm_source=footer), when creating a Mongo adapter for OAuth2, no DSN is sent -- which means that `ZF\Apigility\Admin\Model\AuthenticationModel::createAuthenticationEntityFromConfig()` returns a `null` value, causing a later method call to raise a fatal error.