You need to set the response to empty or what empty means for you in that situation as well. And the response needs to fit the WSDL contract what the response can be.
So in the otherwise add code to set the empty response For example something a like:
<otherwise>
...
<transform>
<constant>NO DATA</constant>
</transform>
</constant>
Though also mind if you use camel-cxf then the dataFormat option can affect how an empty response should be created.