1.0.0 update tries to re-insert existing rows.

It improperly compares against empty string. If tmp
directory wasn't found, it defaults to empty for new
installations and the updater tries to recreate causing
errors.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5702 2fea6a4d-a838-0410-917b-93a53c48e9d2