title: ngrx actions with optional payload
body: <p>I'm working with Angular 8 and NgRx 8, is it possible create an action with optional payload?
  I'm trying to do something like:</p>
  
  <pre><code>export const MyAction = createAction('[Action ] MyAction', props&lt;{ prop?: string }&gt;());
  </code></pre>
  
  <p>but unfortunately doesn't work.
  Could you help me?</p>