cmd/cloner: preserve nil-valued entries when cloning map (#19749)

The codegen path for map-of-slice-of-pointer fields, skipped
nil-valued entries. That dropped the key from the map.

This broke how dns.Config.Routes uses nil values sentinels.

Fixes #19730
Fixes #19732
Fixes #19746
Fixes #19744

Change-Id: Ic6400227f4ab21b3ca0e8c0eeecf9b83d145a9ab

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

net/dns/dns_clone.go

@@ -35,6 +35,7 @@ func (src *Config) Clone() *Config {
 		dst.Routes = map[dnsname.FQDN][]*dnstype.Resolver{}
 		for k, sv := range src.Routes {
 			if sv == nil {
+				dst.Routes[k] = nil
 				continue
 			}
 			dst.Routes[k] = make([]*dnstype.Resolver, len(sv))