config: Fix getting default branch in SCM dict
If user configures branch as explicit None, we want to default to HEAD
(which is most likely refs/heads/master in git).
The original code was getting branch as None, which lead to wrong
resolver being used and the repo url being used as branch.
Fixes: https://pagure.io/pungi/issue/1181
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>