Hide Sidebar when dynamic Menu Items returns null or empty

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [ ] feature request
* [x] question about the decisions made in the repository

### Issue description

**Current behavior:**

When Service in [PagesComponent](https://github.com/akveo/ngx-admin/blob/master/src/app/pages/pages.component.ts) returns empty or null the sidebar still shows.

**Expected behavior:**

What I want to have is the Sidebar must be hiding itself (if that is possible.).

**Steps to reproduce:**

Subscribe PagesComponent to a Service that will return an empty set for menu.

### Other information:

**npm, node, OS, Browser**

Node: 10.14.2
NPM: 6.4.1
OS: Windows 10
Browser: Chrome Version 71.0.3578.98

**Angular, Nebular**
Angular: 7.1.0
Nebular: 3.0.1