network, netstat, test: Use `BeEmpty` matcher
network, netstat, test: Use `BeEmpty` matcher

It is an implementation detail on how an empty list is represented.
In Go, an empty list may be presented as `nil` or as a list with no
items.

The suggested matcher is agnostic to how an empty list is represented.

Signed-off-by: Edward Haas <416ab6a6c3e37a8cab0d365516e79cce8f9be5ad@redhat.com>

kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt