/**
 * Show default short description if empty
 */

add_action('woocommerce_single_product_summary', 'cc_short_description_if_empty', 21);

function cc_short_description_if_empty() {

  global $post;
  if (empty ($post->post_excerpt) ) {
    $post_excerpt = '<p class="default-short-desc">';
    $post_excerpt .= 'This is the default, global, short description.<br>It will show if <b>no short description has been entered!</b>';
    $post_excerpt .= '</p>';
    echo $post_excerpt;
  }

}