Implement empty

Hi,

I have implemented `empty` which was missing. I had to use `_empty` to avoid a naming space clash with 
```
imported from ‘Puppet.PP’ at Puppet/Stdlib.hs:4:1-16
                             (and originally defined in ‘Text.PrettyPrint.ANSI.Leijen’)
```
As a note I only need to do:
```
  $classes = hiera('classes')
  if empty($::role) {
    $catalog_role = $classes
  } else {
    $catalog_role = flatten([$::role, $classes])
  }
```
Maybe you know a better way to do this ?