Fix automate/agents path when not logged in

What does this MR do and why?

Fix automate/agents path when not logged in

If the project was public, this would cause an exception.

Visiting https://gitlab.com/gitlab-org/gitlab/-/automate/agents when not logged in will cause a 500 error. This changes it to a 404.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit http://gdk.test:3000/gitlab-duo/test/-/automate/agents when not logged in
2. Confirm you get a 404
3. Disable Duo features for the project: Project.find_by_full_path("gitlab-duo/test").project_setting.update!(duo_features_enabled: false)
4. You should now again get a 404 error.
5. Enable Duo features for the project: Project.find_by_full_path("gitlab-duo/test").project_setting.update!(duo_features_enabled: trui)
6. Try again on master, and you'll get a 500 when not logged in.

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