Don't bother GROUP_CONCATing tag titles that are empty. Restructure the BOOK_TAG_SQL #define to be somewhat more readable. Test by adding tags to a uri then renaming the tag to empty and making sure they don't show up in autocomplete results.
git-svn-id: svn://10.0.0.236/trunk@257572 18797224-902f-48f8-a5cc-f745e15eee43