Is a dynamic web service response valid?

Is a dynamic web service response valid?

When invoking a web service I get a dynamic response in XML format.
So response could be :
<response>
<test1>test1</test1>
<test2>test1</test2>
<test3>test1</test3>
<test4>test1</test4>
</response>
or :
<response>
<test1>test1</test1>
<test2>test1</test2>
</response>
But I think the response should be static in order for the java class to
be unmarshalled correctly from the xml.
So instead of
<response>
<test1>test1</test1>
<test2>test1</test2>
</response>
This should be :
<response>
<test1>test1</test1>
<test2>test1</test2>
<test3></test3>
<test4></test4>
</response>
This means I can now handle the response and check for missing data.
Am I correct in my thinking ?