Add N/A option to the manager filter and make "N/A" the default for any managed users who have a disabled manager

Today if a manager is disabled in the system, nothing at all happens to the users they manage. These users will still show as being managed by the disabled user. It would be helpful if any managed users of a disabled manager automatically defaulted to "N/A" as their listed manager. In addition, it would then be beneficial to add "N/A" as a Manager filter option on the user's tab in Admin so organizations can quickly identify any users with no manager listed. The disabled manager is not searchable through the manager filter on the users tab, so it can be difficult to identify the users that need their manager to be updated in the system.