Resolve "ActiveRecord::StatementInvalid: PG::NotNullViolation: ERROR: null value in column "a_mode" violates not-null constraint"

What does this MR do?

Some old MR diffs don't have file modes stored, for some reason. In that case, we make one up.

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

I don't think it's absolutely crucial that these modes are correct, as they are only used for display. Still, if any look obviously wrong, please tell me.

Why was this MR needed?

It was preventing a background migration from finishing.

Does this MR meet the acceptance criteria?

• 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

What are the relevant issue numbers?

Closes #36630 (closed).