Fix default branch throwing error instead of null
Created by: eseliger
When getDefaultBranchForRepo discovers a repo is empty or still cloning, it returns "", nil
. The resolver didn't properly account for that though and returned a GitRefResolver even if the refName is empty. That led to a ref resolver that tried to read the repo at repoName@
which is an invalid refspec and causes an error to be thrown. This fixes it.