Show mdmctl config switch usage if no server -name passed (#791)

cmd/mdmctl/config.go

@@ -179,6 +179,11 @@ func switchCmd(args []string) error {
 		return err
 	}
 
+	if flagset.NFlag() == 0 {
+		flagset.Usage()
+		os.Exit(1)
+	}
+
 	return switchServerConfig(*flName)
 }
 func migrateCmd(args []string) error {
@@ -269,6 +274,9 @@ func switchServerConfig(name string) error {
 	if err != nil {
 		return err
 	}
+	if name == "" {
+		return errors.New("bad input: name can't be empty.")
+	}
 	if _, ok := clientCfg.Servers[name]; !ok {
 		return fmt.Errorf("no server named \"%s\" found", name)
 	}