Allow blank destinations

It's possible to have a blank destination. A typical use case is discarding
everything except one domain[1].

example.org :
.example.org :
* discard:

One would implement this using puppet-postfix as follows:

postfix::transport { 'example.org':
  ensure      => present,
  destination => '',
}
postfix::transport { '.example.org':
  ensure      => present,
  destination => '',
}
postfix::transport { '*':
  ensure      => present,
  destination => 'discard',
}

However, this will result into an error:

err: /Stage[main]/MyClass/Postfix::Transport[example.org]/Augeas[Postfix transport - example.org]: Could not evaluate: Save failed with return code false
err: /Stage[main]/MyClass/Postfix::Transport[.example.org]/Augeas[Postfix transport - .example.org]: Could not evaluate: Save failed with return code false

The solution is to use clear instead of set if the destination is empty. This
patch changes the destination to follow a similar pattern as nexthop.

[1]: http://dannyman.toldme.com/2008/09/15/howto-postfix-deliver-external-devnull/