client/tailscale: document ServeConfig accessors a bit more

Updates tailscale/corp#7515

Change-Id: Iecae581e4b34ce70b2df531bc95c6c390a398c38
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/localclient.go

@@ -862,7 +862,7 @@ func (lc *LocalClient) NetworkLockSign(ctx context.Context, nodeKey key.NodePubl
 // SetServeConfig sets or replaces the serving settings.
 // If config is nil, settings are cleared and serving is disabled.
 func (lc *LocalClient) SetServeConfig(ctx context.Context, config *ipn.ServeConfig) error {
-	b, err := json.Marshal(&config)
+	b, err := json.Marshal(config)
 	if err != nil {
 		return fmt.Errorf("encoding config: %w", err)
 	}
@@ -874,13 +874,18 @@ func (lc *LocalClient) SetServeConfig(ctx context.Context, config *ipn.ServeConf
 }
 
 // GetServeConfig return the current serve config.
+//
+// If the serve config is empty, it returns (nil, nil).
 func (lc *LocalClient) GetServeConfig(ctx context.Context) (*ipn.ServeConfig, error) {
 	body, err := lc.send(ctx, "GET", "/localapi/v0/serve-config", 200, nil)
 	if err != nil {
 		return nil, fmt.Errorf("getting serve config: %w", err)
 	}
-	sc := new(ipn.ServeConfig)
-	if err := json.Unmarshal(body, sc); err != nil {
+	return getServeConfigFromJSON(body)
+}
+
+func getServeConfigFromJSON(body []byte) (sc *ipn.ServeConfig, err error) {
+	if err := json.Unmarshal(body, &sc); err != nil {
 		return nil, err
 	}
 	return sc, nil

client/tailscale/localclient_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2022 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.
+
+//go:build go1.19
+
+package tailscale
+
+import "testing"
+
+func TestGetServeConfigFromJSON(t *testing.T) {
+	sc, err := getServeConfigFromJSON([]byte("null"))
+	if sc != nil {
+		t.Errorf("want nil for null")
+	}
+	if err != nil {
+		t.Errorf("reading null: %v", err)
+	}
+
+	sc, err = getServeConfigFromJSON([]byte(`{"TCP":{}}`))
+	if err != nil {
+		t.Errorf("reading object: %v", err)
+	} else if sc == nil {
+		t.Errorf("want non-nil for object")
+	} else if sc.TCP == nil {
+		t.Errorf("want non-nil TCP for object")
+	}
+}