Added checking if there is input and output for operations under bindings section

There are situations where binding operation does not have input and/or output. In this situation module throws following exception:
Caught exception: TypeError: Cannot read property '$name' of null
Example of such operation defined in XML under "wsdl:binding" might be:
<wsdl:operation name="WsServerException">
  <soap:operation soapAction=""/>
  <wsdl:fault name="WsServerExceptionFault">
    <soap:fault name="WsServerExceptionFault" use="literal"/>
  </wsdl:fault>
</wsdl:operation>
Merge pull request #457 from BonneVoyager/master

Added checking if there is input and output for operations under bindings section.