Fix issue with exception when sensor API returns 204 response

See test failure here: http://brooklyn.builds.cloudsoftcorp.com/job/Brooklyn-Master-Build/io.brooklyn$brooklyn-rest-server/174/testReport/junit/brooklyn.rest.resources/ApplicationResourceTest/testReadEachSensor/

Caused by 204 response being returned by Jersey client wrapped in an exception when we have an attribute with a null value.