Workitem query question

I need to create a query in RTC to get all workitems assigned to a person and Assigned to integrator release field is not 0.3. Assigned to integrator release field is an enumeration. For some workitems the Assigned to integrator release field is empty.

I am able to select Assigned to contains first name of the person and for Assigned to integrator release I need to be able to create OR group where all the values are selected except 0.3 or the field Is Null, I don't see the Is Null option in the query. Options available are Is, Is Not and Exists.

Is there a way to create a query to select the records I need ? Any help is much appreciated.

Thank You
NP