Fix running Confidential ContainerPlat

[DomAyre]

Why

We discovered that running ContainerPlat using hcsshim/main was broken

A bisection pointed at this commit 8d81359

Rebuilding and using containerd-shim-runhcs-v1.exe solves the original issue I saw below but presented a new one.

When we parse HostComputeEndpoint into an LCOWNetworkAdapter in convertToLCOWReq() the endpoint has a default route (0.0.0.0/0) where the NextHop field is an empty string, which I believe is invalid, it either needs to be a real gateway or the empty gateway. I traced it back to the construction of the endpoint structure in hcnendpoint.go#getEndpoint() and saw the raw data

{
    …
    "Routes": [
        {
            "NextHop": "",
            "DestinationPrefix": "0.0.0.0/
        }
        …
}

While we might still be able to fix the empty string at source, I think fixing the parsing to ignore invalid routes like this is a suitable defensive approach either way.