Fix an error when no ACLs are specified

Calling strings.Split() on an empty string and a non-null separator
returns a slice with one element...

cmd/auth-server/main.go

@@ -144,6 +144,10 @@ func main() {
 	if *tcpAddr != "" {
 		var llsrv lineproto.Handler = srv
 		if *sslCert != "" {
+			var acls []string
+			if *sslACLs != "" {
+				acls = strings.Split(*sslACLs, ",")
+			}
 			var err error
 			llsrv, err = newTLSServer(
 				llsrv,
@@ -152,7 +156,7 @@ func main() {
 					Key:  *sslKey,
 					CA:   *sslCA,
 				},
-				strings.Split(*sslACLs, ","),
+				acls,
 			)
 			if err != nil {
 				log.Fatal(err)