How to skip a validation if the form is empty?

Hi there, i am creating a validator for a number input. But, if the input is empty, it will still return true. How can i do it ?

The current code is like this :

$rank = new Text("rank");

$rank->setLabel("Rank");

$rank->addValidators(array(

new Numericality(array('message' => 'Rank must be a number'))

));

Thanks in advance :)