Fix migration when project repository is missing

What does this MR do?

Backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9392

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9518

Does this MR meet the acceptance criteria?

• Documentation reviewed by technical writer or follow-up review issue created
• 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