[FIX] sale: fallback on the product uom

`_compute_discount` from subscription add dependencies that triggered
the compute before the `product_uom` was set in a sale order line, once
the uom is set, the `_compute_discount` will be triggered again.

task-2960768

closes odoo/odoo#98780

Related: odoo/enterprise#30721
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>