Unable to drop outbound external mail using transports

I've tried to follow http://dannyman.toldme.com/2008/09/15/howto-postfix-deliver-external-devnull/ with the following:

```
postfix::transport { 'example.org':
  ensure      => present,
  destination => '',
}
postfix::transport { '.example.org':
  ensure      => absent,
  destination => '',
}
postfix::transport { '*':
  ensure      => present,
  destination => 'discard',
}
```

But augeas fails with
```
err: /Stage[main]/Development::Mail/Postfix::Transport[example.org]/Augeas[Postfix transport - example.org]: Could not evaluate: Save failed with return code false
err: /Stage[main]/Development::Mail/Postfix::Transport[.example.org]/Augeas[Postfix transport - .example.org]: Could not evaluate: Save failed with return code false
```