fix: RESTful api missing TunConf.device

In commit 54fee7b, due to failure to take into account that
not all required parameters of `sing_tun.server.New` have
default values provided by `LC.Tun`, the name of the tun device
cannot be obtained when `TunConf.device` is not explicitly
configured. This commit fixed the issue.

listener/listener.go

@@ -84,9 +84,7 @@ type Ports struct {
 
 func GetTunConf() LC.Tun {
 	if tunLister == nil {
-		return LC.Tun{
-			Enable: false,
-		}
+		return LastTunConf
 	}
 	return tunLister.Config()
 }