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

That sounds like a good approach. I'd like to see the implementation.

I did it and it’s working fine, with strings, and also with numeric values. 
Here is my /application/core/MY_Input.php

PHP Code:

class MY_Input extends CI_Input {
    public function post($index = NULL, $xss_clean = NULL)
    {
        $value= $this->_fetch_from_array($_POST, $index, $xss_clean);
        return $value === '' ? null : $value;
        //return isset($value) ? $value : (isset($default_fail) ? $default_fail : NULL);
    }
}