By using list_price_used we are sure to have a list price in the company
currency so there is not need for conversion.
Also list price of a product can be empty so we should not try to round it.
Closes#13110
(grafted from 710927560bfe722fc02c0d22bbdf2c1615f787a7)
--HG--
branch : 7.0
Since e35820296f3a, sum_tree is use to compute project total but it requires
to have a default value for parents that are not in the list of instances.
Closes#12305
Cédric Krier