[FIX] tabs: If all the tabs are hidden don't try and show/hide nonexistent tab content (r48154 made side-by-side full-screen translation fail on every other page load)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48201 b456876b-0849-0410-b77d-98878d47e9d5