Add spec for `/api/internal/discover` without user

What does this MR do?

The behavior for GitLab shell calling /discover when a user is not found is currently untested.

We are relying on this behaviour to make GitLab reply "Welcome to GitLab, Anonymous!".

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer