title: Filter all 'null' values from an Observable<T>
body: <p>I have a service with a subject:</p>
  
  <pre><code>@Injectable() export class UserService() {
      private currentUserSubject = new BehaviorSubject&lt;User&gt;(null);
      public currentUser = this.currentUserSubject.asObservable().distinctUntilChanged(); 
  
      ... // emitting new User    
  }
  </code></pre>
  
  <p>Have a component I inject this service into and subscribing on updates:</p>
  
  <pre><code>@Component() export class UserComponent {
      constructor(private userService: UserService) {
          this.userService.currentUser
              .subscribe((user) =&gt; {
                  // I want to see not null value here
              })
      }
  }
  </code></pre>
  
  <p>I want to apply something to <code>Observable&lt;User&gt;</code> to filter all null values and get into subscribe only when <code>User</code> is actually loaded.</p>