(#1076) Show warning if an empty group is specified

If the user passes an empty group to the user type, puppet will execute
an invalid usermod command.

e.g. if groups of user 'foo' is currently 'bar' and we do

user { 'foo':
  groups     => '',
  membership => minimum # this is the default
}

puppet will build a should value of ',bar' (old group + the new group '') and
will pass '-G ,bar' to the usermod command (which will then fail).

To really specifiy 'no group' the group property does allow an empty
array so we should fail if we find an empty string as a group.