Add proper nil and error handling to SAML login process

While writing the feature that would allow certain Omniauth providers to be marked as external I noticed that there is a scenario where the gl_user method can return nil and if this is not properly checked, it will lead to exceptions that will cause 500 errors. It is quite easy to land in this scenario, so I added nil checks.

I also noticed that the saml method in the omniauth_callbacks_controller.rb file lacked a rescue for Gitlab::OAuth::SignupDisabledError, which can happen if the default configuration from 1_settings.rb is applied. So I also added this check.

/cc @DouweM