[Validator] Fixed NotBlank with booleans

The following code resulted in an error:

````php
class Foo
{
    /**
     * @Assert\Type("bool")
     * @Assert\NotBlank
     */
    public $foo;
}

$foo = new Foo;
$foo->foo = false;
$validator->validate($foo);
````

That's because `empty($value)` returns `true` when `$value` is `false` and it's not put in the exceptions.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -