Symfony2 validator, NotBlank but allow null

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.

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