Resolved with the following:
<?php
$link = get_field( 'supporterlink', get_the_id() );
if ( $link ) : ?>
<a href="<?php echo esc_url( $link ); ?>">
<?php endif; ?>
<img role="img" src="<?php the_post_thumbnail_url( 'medium' ); ?>" alt="<?php the_title(); ?>"/>
<?php if ( $link ) : ?>
</a>
<?php endif; ?>
See updated Gist:
GIST
