New: no-footer class automatically added to the table when no tfoot

- It appears that ti is impossible to know in CSS if a table doesn't
  have a tfoot element reliably and cross browser, so I've added a class
  which is added to the table automatically if the table has no footer
  or the fotoer is empty to complete the styling of the table.

- Fixed error when there are no cells in an empty TR row in the footer.