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>

pungi/checks.py

@@ -305,7 +305,8 @@ def _extend_with_default_and_alias(validator_class, offline=False):
                 and "repo" in instance[property]
             ):
                 instance[property]["branch"] = resolver(
-                    instance[property]["repo"], instance[property].get("branch", "HEAD")
+                    instance[property]["repo"],
+                    instance[property].get("branch") or "HEAD",
                 )
 
         for error in _hook_errors(properties, instance, schema):