Issn example

'2156-537A', Issn::INVALID_CHARACTERS_ERROR],
            ['1119-0231', Issn::CHECKSUM_FAILED_ERROR],
            ['1684-5312', Issn::CHECKSUM_FAILED_ERROR],
            ['1996-0783', Issn::CHECKSUM_FAILED_ERROR],
            ['1684-537X', Issn::CHECKSUM_FAILED_ERROR],
            ['1996-0795', Issn::CHECKSUM_FAILED_ERROR],
        ];
    }

    public function testNullIsValid()
    {
        $constraint = new Issn();

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

        $this->assertNoViolation();
    }

    public function testEmptyStringIsValid()
    {
        $constraint = new Issn();

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