all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

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

ipn/ipnlocal/dnsconfig_test.go

@@ -16,6 +16,7 @@ import (
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/netmap"
 	"tailscale.com/util/cloudenv"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/util/dnsname"
 )
 
@@ -308,10 +309,7 @@ func TestDNSConfigForNetmap(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			verOS := tt.os
-			if verOS == "" {
-				verOS = "linux"
-			}
+			verOS := cmpx.Or(tt.os, "linux")
 			var log tstest.MemLogger
 			got := dnsConfigForNetmap(tt.nm, tt.prefs.View(), log.Logf, verOS)
 			if !reflect.DeepEqual(got, tt.want) {

ipn/ipnlocal/local.go

@@ -71,6 +71,7 @@ import (
 	"tailscale.com/types/preftype"
 	"tailscale.com/types/ptr"
 	"tailscale.com/types/views"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/util/deephash"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/mak"
@@ -3932,10 +3933,7 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	b.dialer.SetNetMap(nm)
 	var login string
 	if nm != nil {
-		login = nm.UserProfiles[nm.User].LoginName
-		if login == "" {
-			login = "<missing-profile>"
-		}
+		login = cmpx.Or(nm.UserProfiles[nm.User].LoginName, "<missing-profile>")
 	}
 	b.netMap = nm
 	if login != b.activeLogin {

ipn/ipnlocal/serve_test.go

@@ -16,6 +16,7 @@ import (
 	"testing"
 
 	"tailscale.com/ipn"
+	"tailscale.com/util/cmpx"
 )
 
 func TestExpandProxyArg(t *testing.T) {
@@ -140,10 +141,7 @@ func TestGetServeHandler(t *testing.T) {
 				},
 				TLS: &tls.ConnectionState{ServerName: serverName},
 			}
-			port := tt.port
-			if port == 0 {
-				port = 443
-			}
+			port := cmpx.Or(tt.port, 443)
 			req = req.WithContext(context.WithValue(req.Context(), serveHTTPContextKey{}, &serveHTTPContext{
 				DestPort: port,
 			}))