Your Mapping has WPDBBY01 mapped as the RFC input, basically the IDoc Root Node. As far as I know this will just send a blank input value to the RFC as the Input and this is why it proabably gets a blank response back.
Right click on WPDBBY01 and select option Return as XML. This will make sure your Entire IDoc is converted into a XML and is mapped to the RFC's Import Parameter- SOURCES as a String
Regards,
Bhavesh