title: Angular 5 - form validation e-mail
body: <p>i want to solve this problem:
  Angular 5 - template driven form</p>
  
  <p>An input-field has type email. Example:</p>
  
  <pre><code>&lt;input type="email" [(ngModel)]="model.email" #email="ngModel" email /&gt;
  </code></pre>
  
  <p>I want to validate this field. But it should not be a required field.
  The validation should only start, if it isn't empty.
  If the field is empty, everthing is fine. Otherwise an error message should be displayed until the e-mail adress is correct.</p>
  
  <p>This is not realy working:</p>
  
  <pre><code>*ngIf="email.untouched &amp;&amp; email.invalid"
  </code></pre>
  
  <p>So, how can i validate the email field? 
  I miss a status like "not empty". </p>
  
  <p>Any hint?</p>