`test_cannot_create_superuser_if_any_user_exists` is flaky

```
=================================== FAILURES ===================================
_______________ test_cannot_create_superuser_if_any_user_exists ________________
[gw0] linux -- Python 3.11.14 /home/runner/.cache/pypoetry/virtualenvs/flagsmith-api-h9Rv37Cj-py3.11/bin/python

admin_user =
api_client =
mocker =

def test_cannot_create_superuser_if_any_user_exists(
admin_user: FFAdminUser, api_client: APIClient, mocker: MockerFixture
) -> None:
# Given
mocker.patch("custom_auth.serializers.is_saas", return_value=False)

email = "test@example.com"
password = FFAdminUser.objects.make_random_password()
register_data = {
"email": email,
"password": password,
"re_password": password,
"first_name": "user",
"last_name": "test",
"superuser": True,
}
url = reverse("api-v1:custom_auth:ffadminuser-list")

# When
response = api_client.post(url, data=register_data)

# Then
assert response.status_code == status.HTTP_400_BAD_REQUEST
> assert response.json()["superuser"] == [
"A superuser can only be created through this endpoint if no other users exist."
]
E KeyError: 'superuser'
```