query for both empty or null values check together

this is my sql query and its work fine in my code but it only check ParentDeptID IS NULL  it not working when ParentDeptID   field is empty how can i check it

("select DeptID,DeptName,(select count(*) FROM HrDept " _           & "WHERE ParentDeptID=sc.DeptID) childnodecount FROM HrDept sc where ParentDeptID IS NULL ", _           objConn)