LengthValidator passes test if string is empty.

Maybe it's just me, but what's the purpose of [this check](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Validator/Constraints/LengthValidator.php#L28):

``` php
if (null === $value || '' === $value) {
    return;
}
```

why returning so early if the string is empty? If I'm checking for a minimum length, shouldn't it add a violation for empty strings too?
