Any empty string in PHP is equal to null, so doing that wouldn’t have the effect we’re going for. It would print the default WP more link: https://developer.wordpress.org/reference/functions/get_the_content/
What we can do is pass a filter so you can filter in your own content more link. I can have this added to the next version.
Find this line: https://github.com/tomusborne/wp-show-posts/blob/1.1.3/wp-show-posts.php#L473
And replace it with this:
<?php
$content_more_link = apply_filters( 'wpsp_content_more_link', false, $settings );
the_content( $content_more_link );
?>Then you can do this:
add_filter( 'wpsp_content_more_link', function() {
return 'Read more';
} );