Get CI details using 'Null' criteria

Get CI details using 'Null' criteria

Hello,

I am just trying to develop a python script to update Warranty Expiry Date for hardware assets. To update this field, I need to get CI names which has no 'Warranty Expiry Date' field is filled. How can I create a xml query to get those CI names? I just tried like below:
<criteria>
            <parameter>
                    <name compOperator="IS">Warranty Expiry Date</name>
                    <value>(null)</value>
            </parameter>
</criteria>

I am using SDP v11.2, and its CMDB API does only support XML input right? 

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