Fill in the value rather than using an empty element.
<model xmlns="http://www.w3.org/2002/xforms" id="p1"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<instance>
<doughnuts xmlns="http://www.example.org/doughnuts/">
<quantity xsi:type="xsd:nonNegativeInteger">12</quantity>
</doughnuts>
</instance>
<submission action="http://www.example.org/formprocessor.php"
method="post" id="q0" />
</model>