Fixes an issue when no amount have been specified while creating an invoice item
Better groovy scriplets (elvis operator)
I believe using BigDecimal for both fields quantity and total can't hurt
Increases size of both fields quantity and total 

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1547719 13f79535-47bb-0310-9956-ffa450edef68