[Refactor]

1.allow maybe empty group
2.use COMPATIBLE(DIRECT alias) when proxy group is empty
3.http provider pass through tunnel

adapter/outboundgroup/selector.go

@@ -50,7 +50,8 @@ func (s *Selector) SupportUDP() bool {
 
 // MarshalJSON implements C.ProxyAdapter
 func (s *Selector) MarshalJSON() ([]byte, error) {
-	var all []string
+	all := make([]string, 0)
+
 	for _, proxy := range getProvidersProxies(s.providers, false, s.filter) {
 		all = append(all, proxy.Name())
 	}