Hi Bruno,
What I understood that your Response XSLT mapping failing because the Response is empty. The Error Log also indicating the same.
You can add one more Message Mapping just above your XSLT, where Sender & Receiver structure would be same (Response Structure of CLICK) and you use mapWithDefault function just to populate the Nodes / Fields (Empty Values) which you are parsing in your next Level XSLT Mapping.
In that way the Input to the XSLT Mapping would be a valid CLICK Structure but without any value. You perform whatever Logic is maintained in your XSLT, but it will not fail for Empty (invalid XML) structure.
Regards,
Nabendu.