title: virtual scroll on Angular 7 is not visible - the height is zero by default
body: <p><strong>Scenario:</strong></p>
  
  <ul>
  <li>Tried a basic virtual scroll test reading this <a href="https://blog.angular.io/version-7-of-angular-cli-prompts-virtual-scroll-drag-and-drop-and-more-c594e22e7b8c" rel="noreferrer">blog post</a></li>
  <li>the array had lots of items</li>
  <li>there was no error</li>
  <li>the list loaded in a virtual scroll but the height of it was 0 by default</li>
  </ul>
  
  <p><strong>quick fix was</strong> to </p>
  
  <ul>
  <li>set the height for cdk-virtual-scroll-viewport to 500px or  set the
  height for class 'example-viewport' in app.component.css</li>
  </ul>
  
  <p><strong>Question</strong>:
  what is the proper way to overcome this zero height?</p>
  
  <p>sample at <a href="https://stackblitz.com/edit/angular-efdcyc" rel="noreferrer">https://stackblitz.com/edit/angular-efdcyc</a></p>