go/upgrade: Add empty upgrade handler

go/upgrade/migrations/empty.go

@@ -0,0 +1,24 @@
+package migrations
+
+const (
+	// EmptyHandler is the name of the upgrade handler that doesn't perform any updates.
+	EmptyHandler = "empty"
+)
+
+var _ Handler = (*emptyHandler)(nil)
+
+type emptyHandler struct{}
+
+func (th *emptyHandler) StartupUpgrade(ctx *Context) error {
+	// Nothing to do.
+	return nil
+}
+
+func (th *emptyHandler) ConsensusUpgrade(ctx *Context, privateCtx interface{}) error {
+	// Nothing to do.
+	return nil
+}
+
+func init() {
+	Register(EmptyHandler, &emptyHandler{})
+}