WIP: Display empty files properly on MR diffs

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23671

What does this MR do?

Adds an "Empty file" message to empty files in the updated merge request diff UI.

I could not see a path to support this feature without adding an addition field to Gitlab::Diff::File. This is my first time working with the GitLab source and I'd love to hear any suggestions for improvements!

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54786

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer