ACF if statement not working when field is empty

I have an ACF field for hero images called hero_image. This field sits at the top of my single.php page like so:

<?php
/**
 * The template for displaying all single posts
 *
 * @link https://developer.wordpress.org/themes/basics/template-hierarchy/#single-post
 *
 * @package sitename
 */

get_header();
?>

<?php 
    $post_id = get_the_ID(); // Required as outside the loop
    $image = get_field('hero_image', $post_id);
    if ($image) {
        echo '<div class="hero">'.wp_get_attachment_image( $image, 'hero').'</div>';
    }
?>

<div class="has-sidebar">

    <div id="primary" class="content-area">

        <main id="main" class="site-main">

        <?php
        while ( have_posts() ) :
            the_post();

            get_template_part( 'template-parts/content', get_post_type() );

            the_post_navigation();

        endwhile; // End of the loop.
        ?>

        </main><!-- #main -->

    </div><!-- #primary -->

    <?php
        get_sidebar();
        get_footer();
    ?>

</div><!-- .has-sidebar -->

I'm using the $post_id variable to fetch the field from outside the loop. The image loads as expected.

If an image hasn't been uploaded for a post using the field, I'm expecting there to be no markup on the front-end. However, I still see the following:

Why isn't my if statement working when the field isn't in use?