title: Rxjs - check if Observable is empty, without subscribing to it
body: <p>I wonder is it possible to check if observable received any value. And I want this without subscribing to it, because I am too lazy to overwrite my current code. Problem is that I create observable in parent component and then passing it to child components. And I need only one tiny thing from it, so subscribing just for that, seems unnecessary...</p>
  <p><strong>parent-component:</strong></p>
  <pre class="lang-js prettyprint-override"><code>adverts$: Observable&lt;advert[]&gt;;
      
  ngOnInit() {
      this.adverts$ = this.advertService.advertsPerUser();
  </code></pre>
  <p><strong>service:</strong></p>
  <pre class="lang-js prettyprint-override"><code>advertsPerUser() {
      return this.advertsKeysPerUser()
          .map(adKeys =&gt; adKeys.map(adKey =&gt; this.afDb.object('adverts/' + adKey.$key )))
          .flatMap(val =&gt; Observable.combineLatest(val) );
  }
  </code></pre>
  <p>So my goal is in parent-component.html do something like:</p>
  <pre class="lang-js prettyprint-override"><code>&lt;div *ngIf=&quot;adverts$&quot;&gt; 
  </code></pre>