Get users only if Gravatar is specified

Hi guys 🙂 !

I would like to retrieve a list of 12 WordPress users, but I would like to have them randomly (at each page reload) AND only 12 users that have a Gravatar specified.

Right now, I have my 12 random users, but I can’t only retrieve those with a Gravatar…
Could you help me ?

Here is what I have right now :

Functions.php :

// ***********************************************************************************
// ******* Random users
// ***********************************************************************************
add_action( 'pre_user_query', 'my_random_user_query' );

function my_random_user_query( $class ) {
    if( 'rand' == $class->query_vars['orderby'] )
        $class->query_orderby = str_replace( 'user_login', 'RAND()', $class->query_orderby );

    return $class;
}

In my template :

<?php $args = array(
  	'role'         => '',
  	'meta_key'     => '',
  	'meta_value'   => '',
  	'meta_compare' => '',
  	'meta_query'   => array(),
  	'include'      => array(),
  	'exclude'      => array(),
  	'orderby'        => 'rand',
  	'offset'       => '',
  	'search'       => '',
  	'number'       => '12',
  	'count_total'  => false,
  	'fields'       => 'all',
  	'who'          => ''
  );

  $users = get_users($args);
  ?>

	<ul class="you-know-people cf">
	  <?php $i=0; ?>
		<?php foreach ( $users as $user ) { ?>

		  <?php $author_avatar = get_avatar($user->ID,55); ?>

				<li class="you-know-people-item <?php if($i%4 === 0){echo 'first';}?>">
					<a href="#">
					<?= $author_avatar; ?>
				</a>
			</li>

			<?php $i++; ?>
		<?php }?>
	</ul>

Thank you in advance,
Cédric