Fix Group list_users function to allow root id (0) to be parsed

This PR can be tested with CLI group subcommand
```
sbesson:dist sebastien$ bin/omero group adduser --id 3 root
Using session 43bcd30e-ec19-4c8d-93ed-40c9df9e9bf6 (root@localhost:4064). Idle timeout: 10.0 min. Current group: system
Added 0 to group 3
sbesson:dist sebastien$ bin/omero group removeuser --id 3 0
Using session 43bcd30e-ec19-4c8d-93ed-40c9df9e9bf6 (root@localhost:4064). Idle timeout: 10.0 min. Current group: system
Removed 0 from group 3
```