fup: don't crash on view if we don't have a cheddar

Classic Go typed-nil bug.

web/fup/cmd/serve.go

@@ -79,9 +79,11 @@ var (
 				AppRoot:             viper.GetString("serve.app-root"),
 				StorageURL:          bucketURL(),
 				RedirectToBlobstore: !viper.GetBool("serve.direct-only"),
-				Highlighter:         highlighter,
 				AuthMiddleware:      fuphttp.TokenAuthMiddleware(viper.GetString("serve.auth.token"), viper.GetString("serve.auth.realm")),
 			}
+			if highlighter != nil {
+				cfg.Highlighter = highlighter
+			}
 			a, err := fuphttp.New(ctx, cfg)
 			if err != nil {
 				return fmt.Errorf("constructing application: %w", err)