ipn: fix the string representation of an empty ipn.Notify

Before: `ipn.Notify}`
After:  `ipn.Notify{}`

Updates #cleanup

Signed-off-by: Alex Chan <alexc@tailscale.com>

ipn/backend.go

@@ -205,8 +205,12 @@ func (n Notify) String() string {
 	}
 
 	s := sb.String()
+	if s == "Notify{" {
+		return "Notify{}"
+	} else {
 		return s[0:len(s)-1] + "}"
 	}
+}
 
 // PartialFile represents an in-progress incoming file transfer.
 type PartialFile struct {

ipn/backend_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipn
+
+import (
+	"testing"
+
+	"tailscale.com/health"
+	"tailscale.com/types/empty"
+)
+
+func TestNotifyString(t *testing.T) {
+	for _, tt := range []struct {
+		name     string
+		value    Notify
+		expected string
+	}{
+		{
+			name:     "notify-empty",
+			value:    Notify{},
+			expected: "Notify{}",
+		},
+		{
+			name:     "notify-with-login-finished",
+			value:    Notify{LoginFinished: &empty.Message{}},
+			expected: "Notify{LoginFinished}",
+		},
+		{
+			name:     "notify-with-multiple-fields",
+			value:    Notify{LoginFinished: &empty.Message{}, Health: &health.State{}},
+			expected: "Notify{LoginFinished Health{...}}",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := tt.value.String()
+			if actual != tt.expected {
+				t.Fatalf("expected=%q, actual=%q", tt.expected, actual)
+			}
+		})
+	}
+}