I'm having trouble validating a value to allow NULL but not an empty string with the Symfony2 validator component.

I've integrated the component in a Silex application and used the Property Constraint target to validate some properties of my Application Entities (not a Doctrine Entity).

I've added this static method to my Entity class to validate name and service\_id on my Entity, problem is that when `service_id` is NULL which should be valid the `NotBlank` constraint kicks in and reports a violation. 

```
static public function loadValidatorMetadata(ClassMetadata $metadata)
{
    // name should never be NULL or a blank string
    $metadata->addPropertyConstraint('name', new Assert\NotNull());
    $metadata->addPropertyConstraint('name', new Assert\NotBlank());

    // service_id should either be a non-blank string or NULL
    $metadata->addPropertyConstraint('service_id', new Assert\NotBlank());
}

```

Bottomline, I'm looking how to allow either a String or NULL as `service_id` but not allow an empty string.

PS: I've also tried the `MinLength(1)` constraint but that allows empty strings unfortunately.