Describe the bug
The authentik_client API published on PyPI fails to parse UserGroup objects that don't have parent_group set. Not all groups will have a parent group, and parsing Group objects works just fine.

To Reproduce
Steps to reproduce the behavior:

1. Import and configure authentik_client in Python 3.12.2
2. Initialize the baseAPI object.
3. Initialize the coreAPI object from baseAPI.
4. Call coreAPI.core_users_list()

Expected behavior
I would expect it to parse the groups without issue, as calling core_groups_list(), which outputs Group objects that may or may not contain parent_group

Screenshots
If applicable, add screenshots to help explain your problem.

Logs
Output of docker-compose logs or kubectl logs respectively

Version and Deployment (please complete the following information):

• authentik version: 2024.2.2
• Deployment: docker-compose
• Python version: 3.12.2
• authentik_client version: 2024.2.2-1710845398

Additional context
A few of my groups do not have a parent group, as they shouldn't or don't need to.