[ticket/11346] Hide "Mark topics read" link when forum is empty.

TOTAL_POSTS and TOTAL_USERS are not defined for viewforum.php.

PHPBB3-11346
Merge remote-tracking branch 'prototech/ticket/11346' into develop

* prototech/ticket/11346:
  [ticket/11346] Hide "Mark topics read" link when forum is empty.