NotBlank should not fail when false is given

The following code resulted in an error:

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

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

For some reason, `NotBlank` is developed to see `false` as blank. I can't see the reason behind this and it would fail in cases like this.
