Do not insist on ifplugd package

bnc#891517
Checking for `nil` is a correct generic replacement for `Builtins.size`, but here `List` is [documented](http://www.rubydoc.info/github/yast/yast-yast2/Yast/NetworkInterfacesClass:List) not to return `nil`, so `if !ifaces.empty?` is enough.