[FIX] account: Set memo to ref if not payment_reference

Issue

	- Install "Accounting" module
	- Create a new bill :
	- Set vendor
	- Set no payment reference
	- Set a bill reference
	- Add products to bill
	- Confirm and click on Register Payment

	Memo field has no value.

Cause

	The memo field is based on lines name.
	Lines name is computed in `_onchange_payment_reference`
	and depends only on payment_referecence.

Solution

	If no `payment_reference`, fallback on `ref` to set line name.

opw-2440389

closes odoo/odoo#66032

Co-author: nboulif <bon@odoo.com>
Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>