User: when creating new user, ignore empty groups value

When trying to create a new user, if I pass a groups parameter of ' ' (empty string), the call to get_groups_set() on library/system/user:283 fails because it checks if ' ' is a valid group.

e.g.
```yaml
- user: name=${item.name} groups=${item.groups}
  with_items:
    - { name: 'joe', groups='wheel' }
    - { name: 'sally', groups='' }
```
The creation of sally will fail.