title: BehaviorSubject with Typescript gives error if initialized with null as default value
body: <p>I have an interface called <strong>SelectedNodeInfoInterface</strong>. I have a BehaviorSubject which I would like to initialize with null and after some operation completes, or method is called i pass new value (.next(value)) on it. It used to work fine but I enabled </p>
  
  <blockquote>
    <p>strict:true </p>
  </blockquote>
  
  <p>in my typescript config and since then it started to give me error. If I disable that flag, it compiles without problem and works as expected. However, I would like to keep strict mode enabled.</p>
  
  <p>Code looks like this</p>
  
  <pre><code>export class SharedLinkedDetailsService {
  selectedAccounts$: BehaviorSubject&lt;SelectedNodeInfoInterface&gt;;
  constructor() {
    this.selectedAccounts$ = new BehaviorSubject(null);
  
   }
  
  accountsSelected(selectedNode: SelectedNodeInfoInterface) {
    this.selectedAccounts$.next(selectedNode);
  }
  
  export interface SelectedNodeInfoInterface {
    selectedAccounts?: string | string[];
    highlightEdges?: boolean;
   }
  </code></pre>
  
  <blockquote>
    <p>Type 'BehaviorSubject' is not assignable to type
    'BehaviorSubject'.</p>
  </blockquote>
  
  <p>Any suggestions will be appreciated.</p>