Rely on the list of branches rather than the ``.empty`` attribute
There is a risk sometime that the git repo is no longer empty (because
we added a specific head for a PR for example) so ``.empty`` returns
False while there is actually no content in the git repo.
In this case, there will be no branches in that git repo.
So use this mechanism to determine if the git repo is empty or not.
Fixes https://pagure.io/pagure/issue/3090
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>