[Validator] Constaint Violation List add isEmpty()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

At http://symfony.com/doc/current/book/validation.html

```php
$validator = $this->get('validator');
$errors = $validator->validate($author);

if (count($errors) > 0) {
    return new Response(print_r($errors, true));
} else {
    return new Response('The author is valid! Yes!');
}
```

While doing this code several times, to me doing a count every time doesn't look good, I would prefer to have a isEmpty()

```php
$validator = $this->get('validator');
$violations = $validator->validate($author);

if ($violations->isEmpty()) {
    return new Response('The author is valid! Yes!');
}

return new Response(print_r($errors, true));
```

The isEmpty() just compare against array dropping the count.

**If this PR is accepted and is wanted I can make a PR to change the doc**