feat(nginx): generate 502 config for apps without web listeners

When an app has no web listeners (not yet deployed, no web process type,
or web processes not running), generate a minimal nginx config that
returns 502 Bad Gateway instead of having no config at all. This ensures
domains resolve, monitoring tools detect non-200 status codes, and SSL
certificate provisioning tools like letsencrypt can function.

plugins/nginx-vhosts/proxy-build-config

@@ -26,7 +26,7 @@ trigger-nginx-vhosts-proxy-build-config() {
       rm -f "${DOKKU_LIB_ROOT}/data/nginx-vhosts/app-$APP/nginx.conf.sigil"
     fi
   else
-    nginx_clear_config "$APP"
+    nginx_build_config "$APP"
   fi
 }