title: check if not completed Observable is empty
body: <p>Is there a good way to check if not completed Observable is empty at that exact time?</p>
  
  <pre><code>let cache = new ReplaySubject&lt;number&gt;(1);
  ...
  // Here I want to know if 'cache' still empty or not. And, for example, fill it with initial value.
  cache.isEmpty().subscribe(isEmpty =&gt; {
      if (isEmpty) {
          console.log("I want to be here!!!");
          cache.next(0);
      }
  });
  // but that code does not work until cache.complete()
  </code></pre>