title: Show message if Observable has no items afters loading
body: <p>On an Angular 7 application I have the following:</p>
  
  <pre><code>&lt;div *ngIf="(posts$ | async)?.length &gt; 0; else loader"&gt;
    &lt;ng-container *ngFor="let post of posts$ | async"&gt;
      posts contents
    &lt;/ng-container&gt;
  &lt;/div&gt;
  &lt;ng-template #loader&gt;
    Loading ...
  &lt;/ng-template&gt;
  </code></pre>
  
  <p>Sometimes <strong>after loading</strong> there are no posts ... </p>
  
  <p>In that case I would like to show a message saying "Posts not found".</p>
  
  <p>How can I do this? </p>