title: Material Datepicker check value is empty
body: <p>I have the material datepicker inside mat-radio-group as one of the options. When the user selects the FFM radio button the datepicker control opens and the user selects the date, but if the user selects the FFM option and not select a date from the datepicker dialog instead clicks somewhere else on the screen then the datepicker input control has no value on it but internally the datechange events assigns the current date. I tried the ngModel option as per this post 
  <a href="https://stackoverflow.com/questions/44356173/angular-2-material-datepicker-value">44356173</a>, but it didn't work. What I am trying to do is, check if the user selected a date from the datepicker dialog or not, if not the form should not be valid.</p>
  
  <pre><code>&lt;mat-radio-group formControlName="installStatus" [(ngModel)]="installStatus"&gt;
      &lt;mat-radio-button value="8555"&gt;8555 - Delay Crew&lt;/mat-radio-button&gt;
      &lt;mat-radio-button value="9000"&gt;9000 - Self Install&lt;/mat-radio-button&gt;
      &lt;mat-radio-button value="FFM" (click)="datePicker.open()"&gt;
          &lt;mat-input-container&gt;
              &lt;input [(ngModel)]="date" matInput [matDatepicker]="datePicker" placeholder="FFM"&gt;
              &lt;mat-datepicker #datePicker&gt;&lt;/mat-datepicker&gt;
          &lt;/mat-input-container&gt;
      &lt;/mat-radio-button&gt;
  &lt;/mat-radio-group&gt;
  
  export class AddsaleComponent implements OnInit {
    date: any;
  }
  </code></pre>