title: Required validator does not work with number
body: <p>When the input is empty, no error appear.</p>
  
  <p>The reason is that the NumberValueAccessor selector match the input, and set the value of the Control to a number. If the entered string of the input does not match a number, the value is NaN. This is the case for the empty string.
  But the required validator return a error only if the control value is blank (null or undefined) or is a empty string. This is not the case of any of these two conditions because the NumberValueAccessor set the value to the NaN number.</p>
  
  <p>The required validator can not really verify the NaN value because it can appear for empty string, but also for string that are not a number (like 'aaa'). The problem is that the validator can not retrieve the entered string from the control variable.</p>
  
  <p>Quoted from <a href="https://github.com/angular/angular/issues/6932" rel="nofollow noreferrer">https://github.com/angular/angular/issues/6932</a></p>
  
  <p>I am still facing this issue. Is there any solution in Angular2</p>