post_grid_loop_no_post

See the code sample here.

add_action('post_grid_loop_no_post', 'post_grid_loop_no_post_20200614', 10, 2);

function post_grid_loop_no_post_20200614($args, $post_grid_wp_query){
    $grid_id = $args['grid_id'];
    $post_grid_options = $args['options'];

    ?>
    <div class="">Custom HTML here</div>
    <?php
}

https://imgur.com/gnCzGQP