Setting fallback (default) image to featured image block

I was trying to make a block based theme and encountered a problem trying to set a default featured image in case some posts do not have them.

Basically, we used to do this in php

<?php if ( has_post_thumbnail() ) { the_post_thumbnail(); } else { ?> <img src="<?php bloginfo('template_directory'); ?>/images/default-image.jpg" alt="<?php the_title(); ?>" /> <?php } ?> 

But now with blocks, themes are html and all I can see is

<!-- wp:post-featured-image {"isLink":true} /--> 

I couldn’t find any other parameters here nor an if/else block so I could just add a plain html.

If anyone has done this, that would be of great help.

Thanks.