Sorry, I don't really catch your concern... As I said, doesn't matter, we call it exactly the same way as we did in 12.1.9, the only difference as you said is that the response is different, we got now the OutputXML (which is the trx output parameter, XML type) in the Row tag, where in the past this tag was simply empty...
So for example, we use this kind of url :
http://<server>:<port>/XMII/Runner?Transaction=test&outputParameter=OutputXML
you can easily reproduce this, create a trx calling a query returning no row, and assign query result to output parameter of the trx...
Regards
Olivier