Resolve "NoMethodError: undefined method `sha' for nil:NilClass"

What does this MR do?

Fix a problem when hidden MR refs didn't exist, the source branch was deleted, and you tried to view the MR.

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

Don't think so - it follows https://gitlab.com/gitlab-org/gitlab-ce/issues/38319#note_41618548 pretty closely.

Why was this MR needed?

It's a regression in 10.0!

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

What are the relevant issue numbers?

Closes #38319 (closed).