[sso-admin] Fix regression on new installations

When introducing typing, we erroneously started passing an empty parent
Id instead of None, and the underlying Keycloak library failed to create
the groups.

Closes #15

dd-sso/admin/src/admin/lib/keycloak_client.py

@@ -321,9 +321,10 @@ class KeycloakClient:
 
     def add_group(self, name : str, parent : str="", skip_exists : bool=False) -> Any:
         self.connect()
+        parentId : Optional[str] = None
         if parent:
-            parent = self.get_group_by_path(parent)["id"]
-        return self.keycloak_admin.create_group({"name": name}, parent=parent)
+            parentId = self.get_group_by_path(parent)["id"]
+        return self.keycloak_admin.create_group({"name": name}, parent=parentId)
 
     def delete_group(self, group_id : str) -> Any:
         self.connect()