Fix discovery in the absence of listen addresses (fixes #4418)

This makes it OK to not have any listeners working. Specifically,

- We don't complain about an empty listener address
- We don't complain about not having anything to announce to global
  discovery servers
- We don't send local discovery packets when there is nothing to
  announce.

The last point also fixes a thing where the list of addresses for local
discovery was set at startup time and never refreshed.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4517

lib/connections/service.go

@@ -504,6 +504,13 @@ func (s *Service) CommitConfiguration(from, to config.Configuration) bool {
 	s.listenersMut.Lock()
 	seen := make(map[string]struct{})
 	for _, addr := range config.Wrap("", to).ListenAddresses() {
+		if addr == "" {
+			// We can get an empty address if there is an empty listener
+			// element in the config, indicating no listeners should be
+			// used. This is not an error.
+			continue
+		}
+
 		if _, ok := s.listeners[addr]; ok {
 			seen[addr] = struct{}{}
 			continue