NVL

SQL

SELECT NVL(CSUSER01.FIRSTNAME, 'Unknown') AS FIRSTNAME01
  FROM CSUSER CSUSER01
 WHERE 1=1

XML query API

<query alias="testReplaceNull" projectedClass="com.taleo.akirademo.entity.Candidate">
    <projections>
        <projection>
            <replaceNull>
                <field path="firstName"/>
                <string>Unknown</string>
            </replaceNull>
        </projection>
    </projections>
</query>