See #6215 and #6233.

When running

$resolver->setOptional(array('foo'));
$resolver->setNormalizers(array(
    'foo' => function (Options $options, $previousValue) {
        return doSomethingWith($previousValue);
    },
));

$options = $resolver->resolve(array());

then the normalizer is currently executed even though the option is missing. Since there is a test that asserts this behavior, we can't change it without breaking BC.

In Symfony 3.0, normalizers should not be executed for missing options.