Null or a valid string

gistfile1.php

<?php
require_once __DIR__ . '/../vendor/autoload.php';

/**
 * Class InputFilterTest
 */
class InputFilterTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @dataProvider provider
     */
    public function testInput(array $data, $expected)
    {
        echo '=========' . "\n";

        $inputFilter = $this->createInput($data);
        $result = $inputFilter->isValid();
        $this->assertSame($expected, $result);

        echo ($result ? 'valid' : 'not valid');
        echo "\n" . '=========';
    }

    public function provider()
    {
        return [
            [['optionalField' => 'value'], true],
            [['optionalField' => ''], false],
            [['optionalField' => []], false],
            [['optionalField' => new \StdClass()], false],
            "This fails :(" => [['optionalField' => null], true],
            [[], false],
            [['optionalField' => 1], false],
            [['optionalField' => -1], false],
            [['optionalField' => 0], false],
        ];
    }

    /**
     * @param array $data
     * @return \Zend\InputFilter\InputFilter
     */
    private function createInput(array $data)
    {
        $notEmptyType = \Zend\Validator\NotEmpty::ALL & ~\Zend\Validator\NotEmpty::NULL & ~\Zend\Validator\NotEmpty::STRING;
        $input = new \Zend\InputFilter\Input('optionalField');
        $input
//            ->setAllowEmpty(true)
//            ->setRequired(false)
            ->setContinueIfEmpty(true)
            ->getValidatorChain()
            ->attach(new \Zend\Validator\NotEmpty($notEmptyType), true)
            ->attach(new \Zend\Validator\StringLength([
                'min' => 2
            ]), true)
            ->attach(new \Zend\Validator\Callback(function ($value) {
                echo 'validator called' . "\n";
                var_export($value);
                echo "\n";
                return true;
            }), true);

        $inputFilter = new \Zend\InputFilter\InputFilter();
        $inputFilter->add($input);

        $inputFilter->setData($data);

        return $inputFilter;
    }
}