Only preempt for network when there is a network
Only preempt for network when there is a network

When examining preemption for networks, only consider allocs that have
networks.

Fixes https://github.com/hashicorp/nomad/issues/5793

burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad