Fix findTagged "no tags" query in bookmarks tutorial.

- There is a bug in the exising findTagged function.  When calling the
  function without any tags specified all bookmarks are returned, rather
  than the bookmarks that have no associated tags.
- The alternative query (tags specified) was also adjusted to use a
  similar approach - specifying the conditional after the join, rather
  than in it, IMHO making the logic a little clearer.