types/empty: add Message, stop using mysterious *struct{}

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/empty/message.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package empty defines an empty struct type.
+package empty
+
+// Message is an empty message. Its purpose is to be used as pointer
+// type where nil and non-nil distinguish whether it's set. This is
+// used instead of a bool when we want to marshal it as a JSON empty
+// object (or null) for the future ability to add other fields, at
+// which point callers would define a new struct and not use
+// empty.Message.
+type Message struct{}