Fix NoMethodError for BlameController

What does this MR do and why?

Contributes to #406808 (closed)

Problem

We are missing the check for empty blob if #page action.

Solution

• Add the missing check
• Update tests

How to set up and validate locally

1. Visit http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/blame_page/main/wrong
2. You should be redirected to the repository tree (before the fix -> 500 error)

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.