ipn/ipnlocal: set default NoStatefulFiltering in ipn.NewPrefs (#12031)

This way the default gets populated on first start, when no existing
state exists to migrate. Also fix `ipn.PrefsFromBytes` to preserve empty
fields, rather than layering `NewPrefs` values on top.

Updates https://github.com/tailscale/corp/issues/19623

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -672,12 +672,14 @@ func TestPrefsFromUpArgs(t *testing.T) {
 			goos: "windows",
 			args: upArgsFromOSArgs("windows"),
 			want: &ipn.Prefs{
-				ControlURL:       ipn.DefaultControlURL,
-				WantRunning:      true,
-				CorpDNS:          true,
-				AllowSingleHosts: true,
-				RouteAll:         true,
-				NetfilterMode:    preftype.NetfilterOn,
+				ControlURL:          ipn.DefaultControlURL,
+				WantRunning:         true,
+				CorpDNS:             true,
+				AllowSingleHosts:    true,
+				RouteAll:            true,
+				NoSNAT:              false,
+				NoStatefulFiltering: "false",
+				NetfilterMode:       preftype.NetfilterOn,
 				AutoUpdate: ipn.AutoUpdatePrefs{
 					Check: true,
 				},