Creates empty class="" if class setting is cleared

If class was once set and gets cleared it creates an emtpy class="".

Change...

// Return if 'class' setting is not available.
if (!isset($settings['class'])) {
  return;
}

to...

// Return if 'class' setting is not available.
if (!isset($settings['class']) || empty($settings['class'])) {
  return;
}