Handle missing admin groups when changing account types
It's similar to !84 (merged) and another fallout of !57 (merged), trying to change the account type from admin to normal user fails if one of the default admin groups is missing from the system
(gnome-control-center:42738): accountsservice-WARNING **: 12:17:35.408: SetAccountType call failed: GDBus.Error:org.freedesktop.Accounts.Error.Failed: failed to set account type: sudo group not found
but the 'sudo' group is found, the issue is that get_admin_groups errors out on any missing group, the propose change fixes the issue