title: asynsPipe produces null as a first value
body: <p>When using async pipe in Angular that don't fire event immediately(http request or any observable with a delay), got first value as <code>null</code>
  Why is it happening? How to avoid that?</p>
  
  <pre><code>&lt;hello [data]="delayedData$|async"&gt; &lt;/hello&gt;
  </code></pre>
  
  <p>First change:</p>
  
  <pre><code>SimpleChange {
     currentValue: null
     firstChange: true
     previousValue: undefined
  }
  </code></pre>
  
  <p>Second change:</p>
  
  <pre><code>SimpleChange {
     currentValue: 'some real data'
     firstChange: false
     previousValue: null
  }
  </code></pre>
  
  <p>Example on stackblitz:
  <a href="https://stackblitz.com/edit/http-async-pipe-crxm32" rel="nofollow noreferrer">https://stackblitz.com/edit/http-async-pipe-crxm32</a></p>