Fix validation bug creating invalid custom system roles

Fixes a traceback you'll see with the tests here in devel.
ERROR    django.request:log.py:241 Internal Server Error: /api/v1/role_definitions/
Traceback (most recent call last):
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/django/views/decorators/csrf.py", line 56, in wrapper_view
    return view_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/viewsets.py", line 124, in view
    return self.dispatch(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/mixins.py", line 18, in create
    serializer.is_valid(raise_exception=True)
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/serializers.py", line 223, in is_valid
    self._validated_data = self.run_validation(self.initial_data)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/venvs/awx/lib64/python3.12/site-packages/rest_framework/serializers.py", line 445, in run_validation
    value = self.validate(value)
            ^^^^^^^^^^^^^^^^^^^^
  File "/home/alancoding/repos/awx/testing/django-ansible-base/ansible_base/rbac/api/serializers.py", line 147, in validate
    validate_permissions_for_model(permissions, content_type)
  File "/home/alancoding/repos/awx/testing/django-ansible-base/ansible_base/rbac/validators.py", line 117, in validate_permissions_for_model
    raise ValidationError({'permissions': f'Permissions for model {role_model._meta.verbose_name} needs to include view, got: {display_perms}'})
                                                                   ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute '_meta'
Not actually a huge deal (validation error). But a bug.
ansible / django-ansible-base

Fix validation bug creating invalid custom system roles #532

Quality Gate passed
