DNS inconsistency issues

[keeshux]

Regression in Passepartout 1.4.0 from 1.3.0.

The recent refactoring of ConfigurationParser brought a couple issues:

1. dnsServers defaults to [] rather than nil, but this was actually like before.

2. However, the new code in TunnelKitProvider.bringNetworkUp(...) was affected by 1 because:

let dnsServers = cfg.sessionConfiguration.dnsServers ?? reply.options.dnsServers

didn't check for dnsServers.isEmpty. So basically, if cfg.sessionConfiguration.dnsServers was non-nil but empty, it overrode the server-pushed DNS reply.options.dnsServers.

Simple fix: default dnsServers to nil when none is set.

Thorough, backwards-compatible fix:

var dnsServers = cfg.sessionConfiguration.dnsServers
if dnsServers?.isEmpty ?? true {
    dnsServers = reply.options.dnsServers
}