mapper, app: ship MagicDNS Routes as empty slices, not nil

policyChangeResponse already includes everything else; carry DNSConfig
too so the client's netmap DNS is anchored on every policy change
rather than relying on the previous snapshot.

Send the MagicDNS root domains as empty non-nil Resolver slices instead
of nil values. tailcfg.DNSConfig.Clone and net/dns.Config.Clone in
tailscale drop map entries whose value is nil (tailcfg_clone.go and
dns_clone.go both contain `if sv == nil { continue }`). On a major
LinkChange the client's wgengine handler clones lastDNSConfig and
re-applies it; with nil values the cloned config has Routes:{}, dns.Set
wipes Nameservers in /etc/resolv.conf, and curl-by-FQDN fails until
the next route-changing netmap, typically about six minutes later.
Empty slice survives Clone and carries the same "resolve locally"
semantics for Routes entries.