title: Correct way to bind mat-radio-group to nullable boolean?
body: <p>I have a simple form showing data from a REST server.  One of the fields is a boolean but can be null.  My UI has a mat-radio-group with two mat-radio-button elements, one with value "true" and one with value "false".  I would expect that if it was null, neither would be on, and clicking one would set the value to true or false.  However, it doesn't do anything at all.</p>
  
  <p>I tried using ng-value instead of value, but that didn't have any effect.</p>
  
  <pre><code>      &lt;mat-radio-group [(ngModel)]="canBeTrueFalseOrNull"&gt;
          &lt;mat-radio-button value="true"&gt;yes&lt;/mat-radio-button&gt;
          &lt;mat-radio-button value="false"&gt;no&lt;/mat-radio-button&gt;
        &lt;/mat-radio-group&gt;
  </code></pre>
  
  <p>I supposed a can transform the data from the server into strings and then change it back again when I update it, but that seems overly complex.  Is there an easier way to do this?</p>