Search result condition for Null values

Hi, I am trying to set a Search Condition in the Search results Webpart in the general tab. What I am trying to achieve is to exclude from the result all the linked pages to avoid duplications (where NodeLinkedNodeID is null). I am trying something like this: +NodeLinkedNodeID:(Null) or +NodeLinkedNodeID:("") or -NodeLinkedNodeID ... but I don't get the result expected. How can I filter where a field is null with the Search filter Condition annotation?