[Rails5] Fix `Route source can't be blank`

Note: add rails5 to a branch name to trigger rails5 specific CI jobs.

https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/67501055

  1) Projects::CreateService#execute repository creation synchronously creates the repository
     Failure/Error: expect(project).to be_valid
       expected #<Project id: user559/gitlab> to be valid, but got errors: Route source can't be blank
     # ./spec/services/projects/create_service_spec.rb:167:in `block (3 levels) in <top (required)>'