[Soap\Client\DotNet][FIX] Undefined property in void return

The dotNet soap implementation send the result in a [LastRequest]Result xml node, so the DotNet _preProcessResult return directly this node.

But in some case, when the method return anything and if this behaviour is not defined in the ws defition (wsdl for example), this behaviour can rise an notice error because the searched node does not exists.

So I have added an exists condition on the [LastRequest]Result xml node to avoid this error.

The code is not unit-tested because I don't know how to test this peculiar behaviour, but the modification is really really tiny.