title: RxJS - Test for empty Observable array
body: <p>I am learning Angular 2 and RxJS at the moment and feel that this is probably the simplest question there is. I am using the Store to save 'users' to. I want to test to see if this is empty or not from the Typescript side rather than in the Angular template. </p>
  
  <p>Below are the pertinent lines of code. </p>
  
  <pre><code>public users: Observable&lt;User[]&gt;;
  public store: Store&lt;AppState&gt;;
  
  this.users = this.store.select(state =&gt; state.user);
  
  if(!this.users) {
  //DO THIS 
  }
  </code></pre>
  
  <p>I have tried length, == null, == undefined and can't find how to test for this seemingly basic state. Any help appreciated. </p>
  
  <p>Note that saving and retrieving users to the store is working correctly. </p>