Always return MR diff_refs if importing

What does this MR do?

This MR fixes a bug that prevented project import with diff notes in merge requests to be fully migrated. Merge requests in the migrated project would not be shown.

More details in: https://gitlab.com/gitlab-org/gitlab-ce/issues/63955

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content