Apply omniauth defaults when no arguments are given

What does this MR do and why?

This fixes the issue that is blocking #351331 (closed) and preventing us from enabling omniauth_login_minimal_scopes in production.

This issue was caused by our failure to apply default arguments if the configuration does not supply any arguments at all.

Fixes: #353864 (closed)

How to set up and validate locally

Configure a GitLab omniauth provider with no arguments (i.e. where the server is the production instance of GitLab.com).

Try and log in with that provider, and verify that the gl_auth_type parameter is set during authorization.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.