Fixed list_price and sale_price

list_price and sale_price field are numeric, so they don't need quote.
Added extra check on empty value to be converted as 0 .
Added similar check on mysql driver.

[joomlacode tracker](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29551)


Eng. Gabriele Pongelli.