title: Disable 3rd state in Angular Material MatSortable
body: <p>There are three states in sort direction of MatSortable. Is there a way to disable the 3rd state? It has <code>'asc' | 'desc' | ''</code>, id like to only have <code>'asc' | 'desc'</code> available. I'm currently filtering the sort direction, but I'm wondering if this is intuitive enough from a users perspective to not seem like a bug even though the header does display an arrow with current sort direction(see images below).</p>
  
  <ul>
  <li><p><a href="https://material.angular.io/components/table/overview" rel="noreferrer">material.angular.io</a></p></li>
  <li><p><a href="https://material.angular.io/components/table/overview" rel="noreferrer">mat-table</a></p></li>
  </ul>
  
  <p>Oninit lifecycle hook I'm setting a sort default, thought setting disableClear to true would fix this but no-go. Any help appreciated!</p>
  
  <pre class="lang-js prettyprint-override"><code>defaultSort: MatSortable = {
    id: 'displayName',
    start: 'asc',
    /**
     * Whether to disable the user from clearing the sort by finishing the sort direction cycle.
     * May be overriden by the MatSortable's disable clear input.
     */
    disableClear: True
  };
  
  ngOnInit() {
    this.sort.sort(this.defaultSort);
  
    this.sort.sortChange.pipe(
      filter(sort =&gt; !!sort.direction),
      switchMap(sort =&gt; {
        // makes API request only with an actual direction.
      })
    );
  }
  </code></pre>
  
  
  
  <p>No sort direction:
  <a href="https://i.stack.imgur.com/QDu1f.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QDu1f.png" alt="No sort direction"></a></p>
  
  <p>With sort directions:
  <a href="https://i.stack.imgur.com/MVSDj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/MVSDj.png" alt="asc"></a>
  <a href="https://i.stack.imgur.com/QhuEU.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QhuEU.png" alt="enter image description here"></a></p>