title: p-inputMask ng-model not clearing for invalid value
body: <p>I am using <code>inputMask</code> field from PrimeNG</p>
  
  <pre><code>&lt;p-inputMask name="maskValue" [(ngModel)]="attr.attributeValue" mask="aa/99" placeholder="aa/99"&gt;&lt;/p-inputMask&gt;
  
  {{attr.attributeValue}}
  </code></pre>
  
  <p>But if I enter <code>a</code> in the textbox and focus out, that textbox is clearing but ng-model <code>attr.attributeValue</code> is still hold value <code>a_/__</code>. How could we clear the ng-model as well ?</p>