[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 nothing and if this behaviour is not defined in the ws defition (wsdl for example), a notice error is raised 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.