MULE-7326: Can't use WebService consumer with operation without parameters

Changed behaviour of WS Consumer: if the operation to be consumed requires no input parameters, then the payload will be ignored and a request with one empty XML element will be sent (the one required by the operation).
Merge pull request #418 from mulesoft/MULE-7326-2

MULE-7326: Can't use WebService consumer with operation without parameters