Fix undefined method error for invalid ref

What does this MR do and why?

Contributes to #468695 (closed)

Problem

Refactoring from !155245 (merged) introduced a change to the execution flow.

Solution

• Restore the previous flow
• Add a test to check for this error

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Visit link like: http://gdk.test:3000/gitlab-org/gitlab-test/-/blame/%0A
2. You should see 404 page after the fix