Resolve "Error 500 trying to view a merge request JSON: undefined method `binary?' for nil:NilClass"

What does this MR do?

Fix viewing diffs (typically from old MRs) where neither blob exists.

Are there points in the code the reviewer needs to double check?

I think the comments cover it.

Why was this MR needed?

There's a bug that's triggered by our new batch loading.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #41468 (closed).