[Review] Request from 'dmajda' @ 'SUSE/machinery/users-groups-uid-gid-null'

Please review the following changes:
  * 63b61d7 UsersInspector: Improve comment
  * fb34b15 Groups: Use null instead of empty string to indicate missing GID
  * 24e8389 Users: Use null instead of empty string to indicate missing UID/GID