nixos/users-groups: fix confusing error message

If we include users with unset groups, we get this very confusing
message, with invalid Nix code:

       - The following users have a primary group that is undefined: qyliss
       Hint: Add this to your NixOS configuration:
         users.groups. = {};

We don't need to include such users in this check, since they'll be
caught anyway by this one:

       - users.users.qyliss.group is unset. This used to default to
       nogroup, but this is unsafe. For example you can create a group
       for this user with:
       users.users.qyliss.group = "qyliss";
       users.groups.qyliss = {};