Add Unknown for missing VCS class (#11398)

* Add Unknown for missing VCS class

* Fix my PR

* Fix lint

* Update common

Author: ericholscher

readthedocs/projects/models.py

@@ -1242,7 +1242,10 @@ def get_default_branch(self):
         if self.remote_repository and self.remote_repository.default_branch:
             return self.remote_repository.default_branch
 
-        return self.vcs_class().fallback_branch
+        vcs_class = self.vcs_class()
+        if vcs_class:
+            return vcs_class.fallback_branch
+        return "Unknown"
 
     def add_subproject(self, child, alias=None):
         subproject, _ = ProjectRelationship.objects.get_or_create(