This disables the attempt to return an automatically generated excerpt in the scenario where one isn’t manually set.

add_filter('wp_trim_excerpt', 'disable_auto_excerpt', 10, 2 );

// Return only the raw excerpt text if one is manually set.
  function disable_auto_excerpt($text, $raw_excerpt) {
      if ($raw_excerpt) {
        return $text;
      }
      else {
        return '';
      }
  }