The following error message in SOAPMessageComposer could be a bit better:

// check to see if the payload is null or it's a full SOAP Message
if (message.getContent() == null) {
   throw new SOAPException("Null response from service");
}

I would say "Null response from service" is a bit misleading in this case. We should consider one of the following two alternatives:

1) support an empty SOAP body if payload is null
2) generate a more meaningful error, e.g. "unable to create SOAP Body due to null message content"

I suggest #2.

See forum thread for more info.