get_posts – check if custom field has content?

Try this:

$args2 = array (
    'numberposts' => -1,
    'post_type'   => 'houses'
);
$custom_posts2 = get_posts($args2);
foreach ( $custom_posts2 as $custom_post2 ) {

// check here if custom post type opjects custom field "sz_website" has content
$sz_website = get_post_meta( $custom_post2->ID, 'sz_website', true );

    if ( !empty ($sz_website)) {
        echo '<div class="wpcf7-list-item-field">' . $sz_website . '</div>';
    } else { 
        echo "do nothing"; 
    }

}
wp_reset_postdata();

Let me know if helps.