Filter projects without repository in v3/:namespace/repos endpoint

The v3/:namespace API is used by Jira to fetch branch and commit data for the DVCS connector. Jira assumes that every project returned under v3/:namespace/repos has a repository. To avoid API calls that return 404 we should only return projects with repositories.

The endpoint is implemented in lib/api/v3/github.rb#L115-129. We could filter projects with projects = projects.reject(&:empty_repo?)