Fix divison by 0 issue in debug plugin

If you send an empty DB query (thus strlen($query) = 0) you get a division by 0. With this little change, the string length is always at least 1. Since this is onyl to roughly guess if a query is good or bad, changing the result of this by +1 doesn't have a real impact. This will also mean, that these queries get a high value and show up as bad. And I think we can all agree that an empty query is simply a waste of perfectly good CPU cycles. ;-)

This is a replacement for PR #1539 by @elinw Thank you!

Tracker item is here: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32441&start=0