title: Showing mat-autocomplete only when the input has at least one digit
body: <p>Today in my code, when the focus is in the input, all the options are loaded. I want to make the <strong>mat-autocomplete</strong> visible only when the input has at least one digit.</p>
  
  <p>My code:</p>
  
  <pre><code>&lt;mat-form-field&gt;
      &lt;input type="text" placeholder="{{ 'client' | translate }}" matInput [formControl]="entitySearchCtrl" [(ngModel)]="selectedClientForSearch" [matAutocomplete]="auto"&gt;
      &lt;mat-autocomplete #auto="matAutocomplete" [displayWith]="displayFn"&gt;
        &lt;mat-option *ngFor="let client of relatedClients | async" [value]="client"&gt;
          {{ client.socialName }} - {{ client.cnpj_cpf }}
        &lt;/mat-option&gt;
      &lt;/mat-autocomplete&gt;
    &lt;/mat-form-field&gt;
  </code></pre>