gistfile1.php

$factory = new \Zend\InputFilter\Factory();
$i = $factory->createInputFilter(array(
    'password' => array(
        'name'       => 'password',
        'required'   => true,
        'validators' => array(
            array(
                'name' => 'not_empty',
            ),
            array(
                'name' => 'string_length',
                'options' => array(
                    'min' => 8
                ),
            ),
        ),
    ),
));

$i->setData(array('password' => 'test'));

var_dump($i->isValid()); // false

// this is not required, but also may help, 
// if you haven't got any "password" in your data it will be valid. But if it in data, it would not
$i->get('password')->setRequired(false); 


// remove all validators
$i->get('password')->setValidatorChain(new \Zend\Validator\ValidatorChain());

var_dump($i->isValid()); // true