MigrateApproverToApprovalRulesInBatch sidekiq job GRPC::NotFound error

Summary

MigrateApproverToApprovalRulesInBatch was scheduled to walk through all MergeRequest. A very small number of those jobs failed with GRPC::NotFound error. This was caused by jobs needing to fetch code owners, yet the MR's project no longer has a repository.

Possible fixes

Will be fixed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9267