Accept null value on parent column in multipart relationship
Set [OwnerId] on OwnerID to allow nulls.
Put a null value record in your Category table:

INSERT INTO Category (OwnerId,Title) SELECT NULL,'blah??'


I also added  to your category table:

CONSTRAINT UC_OwnerID UNIQUE(OwnerID) 


this is what I have working for you in SQL Fiddle http://sqlfiddle.com/#!3/ac720/2.