Mail Sender: no header when empty

As of yet, Sender header is **always** written in a mail message sent with Stmp or Sendmail, becase it always returns a non-empty string, and so `Zend\Mail\Headers::toString()` can't skip it.

Despites other headers, Sender is always created because of this calls:

* `Zend\Mail\Transport\Smtp::prepareFromAddress line 252`
* `Zend\Mail\Transport\Sendmail::prepareParameters line 254`

With this PR, just like `Zend\Mail\Header\AbstractAddressList`, now Sender will return empty string if no sender address is specified, and during headers creation, it can be skipped.