As has been brought up multiple times (e.g. #11943, #11083, #7412), it's confusing that the string validation constraints ignore empty strings. Should we change this behavior with Symfony 3.0?

• Remove Blank/NotBlank
• Remove if ('' === $value) { return; } from all constraint validators

A consequence would be that for manually validating form submissions, one will have to pre-process the $_POST array before the validation and convert empty strings to null manually:

$formValues = array_replace_recursive($defaultValues, $_POST['form']);
array_walk_recursive($formValues, function (&$value) { if ('' === $value) { $value = null; } });

$validator->validate($formValues, new Collection(array(
    'name' => array(
        new NotNull(),
        new Length(array('min' => 3)),
    ),
    // ...
));

See also the related discussion about separating the type validation from constraint validators in #10221.