When I POST an empty form field the validator return error and break the process.

This is one of the variation I tried:

$this->add($factory->createInput(array(
    'name'       => 'based_on_activity_id',
    'required'   => false,
    'allow_empty'=> true,
    'validators' => array(
        array(
            'name' => 'Digits',
            'options' => array(
            'allowEmpty' => true,
            'required'   => false,
                'messages' => array(
                    'notDigits' => "based_on_activity_id accept only integers",
                ),
            ),
        ),
    ),
)));