app/launcher: ignore nil IPs
app/launcher: ignore nil IPs

Client code will often just parse an address in the format `host:port`
and pass the IP into `WaitForNetworkReady` so if the `host` part was
empty we would receive a nil IP. For a nil IP we don't have to wait
because the bin should succeed.

GitOrigin-RevId: 12c500d915cddb3c88664cb4189ea91a2ee9dfbf

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion