fix(xo-server/RestApi): don't fail on missing express

Which is the case when instanciated from `xo-server-recover-account`.

packages/xo-server/src/xo-mixins/rest-api.mjs

@@ -49,6 +49,13 @@ const subRouter = (app, path) => {
 
 export default class RestApi {
   constructor(app, { express }) {
+    // don't setup the API if express is not present
+    //
+    // that can happen when the app is instanciated in another context like xo-server-recover-account
+    if (express === undefined) {
+      return
+    }
+
     const api = subRouter(express, '/rest/v0')
 
     api.use(({ cookies }, res, next) => {