[FIX] hr_expense: unit_amount erase when set product_id

When an expense is create from email, if the product is not set, when you want set the product it is erase the unit price.

closes odoo/odoo#50397

X-original-commit: 74736914
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>