getInvalidIsbn13 example

        return array_merge(
            self::getValidIsbn10(),
            self::getValidIsbn13()
        );
    }

    public static function getInvalidIsbn()
    {
        return array_merge(
            self::getInvalidIsbn10(),
            self::getInvalidIsbn13()
        );
    }

    public function testNullIsValid()
    {
        $constraint = new Isbn(true);

        $this->validator->validate(null, $constraint);

        $this->assertNoViolation();
    }