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>
```