title: select option with default value undefined with required attribute does not work but value empty
body: <p>I have a dropdown of countries</p>
  
  <pre><code>&lt;select  #countryInput  name="country" [(ngModel)]="room.countryId"   required&gt;
    &lt;option   [selected] [value]="undefined"&gt;Select Country&lt;/option&gt;
    &lt;option *ngFor="let c of countries" [value]="c.id"&gt;{{c.name}}&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>In order to "Select Country" option, it is required to set [value]="undefined" otherwise it would not be shown as default selection instead empty selection.</p>
  
  <p>Even though field marked is as required, on form submit required wont be show as it does for </p>
  
  <pre><code>&lt;option value=""&gt;Select Country&lt;/option&gt; 
  </code></pre>
  
  <p>which is default behaviour in html 5:</p>
  
  <p><a href="https://i.stack.imgur.com/Zktw0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Zktw0.png" alt="enter image description here"></a></p>
  
  <p>As a work around handle validation on form submit, but this time first required fields will be validated but lastly country field.</p>
  
  <pre><code>if (this.room.country == undefined) {
     alert('select country ');
     return false;
  }
  </code></pre>
  
  <p>What could be solution to field with [value]="undefined" and show "Please select an item first" alert?</p>
  
  <p>See the <a href="https://stackblitz.com/edit/angular-select-example-hbcwgo" rel="nofollow noreferrer">forked fiddle</a> from Daniel's</p>