[FIX] purchase_stock: price diff on vendor bill

Steps to reproduce:

• Create Record: Product Category
- Costing Method: Average
- Inventory Valuation: Automated
- Specify a price difference account (may create new account).
- Ensure Stock Interim (Received) account is reconcilable.

• Create Record: Product
- Product Type: Storable
- Cost Price: 1.01
- Product Category: New Product Category

• Create bill with this product, for a quantity of 10.5

-> The is price difference of 0.1

Issue comes from this commit 286ea6b6
which doesn't round if there is a discount.

With this commit we isolate the latter case in order to keep the
price unit rounded if there is no discount.

closes odoo/odoo#87666

Signed-off-by: Florian Gilbert <flg@odoo.com>