[FIX] sale_stock: correctly display lot's quantities on invoice

Current behavior :
Quantity displayed on 'Detailed operations' block for a product lot isn't correctly displayed on the invoice sometimes.

Steps to reproduce : 
- Install Sales and Stock
- Enable 'Units of Measure' in Settings
- Enable debug mode then go to Users and enable 'Display Serial & Lot Number on Invoices' for current user (i.e. Mitchell Admin)
- Create a new product
  - Tracking set to 'By Lots'
  - UoM set to 'g'
- Create a quotation with the previously created product and set the quantity to 3.07
- Confirm the quotation, go to the Delivery and confirm it too
- Go back to the SO and create the invoice, then go to the invoice and confirm it
- Print the invoice

OPW-2704307

closes odoo/odoo#81313

Signed-off-by: Arnold Moyaux <arm@odoo.com>