Applying if (get_post_meta) to new shortcode

First up the display_ad function should have a parameter $content that contains the post’s content upto the point where the shortcode is placed. So you concatenate that $content with your advertisement image before returning it.

Second what’s wrong with :

if( empty( get_post_meta( $post->ID, 'wpcf-display-ad', true ) ) ) {
   return $content;
 } else {
    return $content . $ad_image;
 }