build, lib: Correct total test coverage calculation

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3483

lib/relay/client/empty_test.go

@@ -0,0 +1,10 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// The existence of this file means we get 0% test coverage rather than no
+// test coverage at all. Remove when implementing an actual test.
+
+package client