Fix OAuth bug related to openid_connect

What does this MR do and why?

Related to #424800 (closed)

The bug was introduced by !95308 (merged) but was only uncovered once !95308 (merged) was merged recently because that MR added a boot-time call to the buggy method. Previously the bug would have only been uncovered in very specific runtime circumstances (using openid_connect and attempting to load configuration for an otherwise unconfigured provider).

This change ensures the method fails safely and also updates tests to ensure the issue is caught in the future. I fixed this ASAP because it caused me issues on GDK upgrade and I imagine it will affect others, too.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure openid_connect in gitlab.yml. Be sure not to configure a name under args:

   omniauth: 
     providers:
     - { name: 'openid_connect' }

2. Start GDK ensuring the latest updates from master to include !95308 (merged)
3. Observe the following error on boot:

   GitlabSettings::MissingSetting: option 'name' not defined
   /Users/dblessing/development/gdk/gitlab/lib/gitlab_settings/options.rb:159:in `method_missing'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/auth/o_auth/provider.rb:72:in `block in config_for'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/auth/o_auth/provider.rb:71:in `each'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/auth/o_auth/provider.rb:71:in `find'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/auth/o_auth/provider.rb:71:in `config_for'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/github_import/markdown_text.rb:36:in `github_url'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/github_import/attachments_downloader.rb:15:in `<class:AttachmentsDownloader>'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/github_import/attachments_downloader.rb:5:in `<module:GithubImport>'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/github_import/attachments_downloader.rb:4:in `<module:Gitlab>'
   /Users/dblessing/development/gdk/gitlab/lib/gitlab/github_import/attachments_downloader.rb:3:in `<main>'
   /Users/dblessing/development/gdk/gitlab/lib/tasks/gitlab/db.rake:505:in `block (4 levels) in <main>'
   /Users/dblessing/development/gdk/gitlab/lib/tasks/gitlab/db.rake:586:in `block (4 levels) in <main>'

MR acceptance checklist

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