title: mat-autocomplete show the list only on search text
body: <p>I am building an app and I have the mat-autocomplete. I wanted to show the searched list only on  entering certain text in the input text field. I tried certain ways but no luck</p>
  <pre><code> &lt;mat-form-field class=&quot;example-full-width&quot; appearance=&quot;fill&quot;&gt;
          &lt;mat-label&gt;Assignee&lt;/mat-label&gt;
          &lt;input type=&quot;text&quot; matInput [formControl]=&quot;myControl&quot; [matAutocomplete]=&quot;auto&quot;&gt;
          &lt;mat-autocomplete #auto=&quot;matAutocomplete&quot; [displayWith]=&quot;displayFn&quot;&gt;
            &lt;mat-option *ngFor=&quot;let option of filteredOptions | async&quot; [value]=&quot;option&quot;&gt;
              {{option.name}}
            &lt;/mat-option&gt;
          &lt;/mat-autocomplete&gt;
        &lt;/mat-form-field&gt;
  </code></pre>