title: Material-UI show letter avatar when ther is no image
body: <p>I am using Material-UI Avatar React component  for show  profile images.I need to show letter avatar when there is no image from given url. How can I achieve this?.</p>
  
  <pre><code>&lt;Avatar src={ProfileImageUrl} className={classes.avatar}&gt;
     {userDetails.fistname.charAt(0)+" "+userDetails.lastname.charAt(0)}
  &lt;/Avatar&gt;
  </code></pre>