how to filter null lookup field

before on the Case entity we dont have field_PersonServiceId and now i want to get all of the Case without the field_PersonServiceId

and i try code like this

var filter = "?$filter=field_Patient/Id eq guid'" + patientId + "' and field_PersonServiceId/Id eq guid'" + null + "'";

and i got an error

Unrecognized 'Edm.Guid' literal 'guid'null''