Can I convert empty strings to NULL with a Codeigniter validation rule?

I would like to convert all my POST values from '' to NULL automatically with a Codeigniter validation rule. Is it possible?
Here is my helper, which is working fine with text and numbers :

PHP Code:

function empty2null($value)
{
    return $value === '' ? null : $value;
} 


Can I define it as a validation rule (prepping?) somewhere? How to do this?
Afterwards I would like to send it as a proper NULL value to Doctrine.

I tried to implement it as a classic rules, but this custom function is not triggered when the POST value is empty.

Thank you