exactly, if the hint is '' or ':' then it must be no-namespace, if it is null its all up to the implementation. see https://github.com/phpcr/phpcr/blob/master/src/PHPCR/NodeInterface.php#L131

i should update the api docs at phpcr.github.com but i am blocked by waiting for answers in https://github.com/phpDocumentor/phpDocumentor2/pull/728