Unsupported roles show as empty in the roles assignment list

[mariajgrimaldi]

Description

When authz is enabled, some roles assigned outside of the admin console show up as empty rows in the assignment list.

For example, if a role like Beta Tester is assigned from the instructor dashboard, the assignment appears in the admin console, but the Role column is empty.

Steps to reproduce

1. Enable authz.
2. Go to a course as an admin for the course.
3. Open the instructor dashboard.
4. Assign a role that is not currently supported by the Roles and Permissions Management page, for example Beta Tester.
5. Go to the Roles and Permissions Management page.
6. Open the Team Members tab.
7. Look for the user who received the role.

Expected behavior

The assignment list should clearly show the assigned role, or otherwise handle unsupported roles in a clear way.

Actual behavior

The assignment appears in the list, but the Role column is empty. This makes it hard to understand what role the user has for that scope or why is the user even listed.

[mariajgrimaldi]

This might be related to openedx/openedx-authz#353 & openedx/openedx-authz#354 since all comes to unsupported roles.