[FIX] point_of_sale: do not reconcile 0 sum lines

- Have a clean V13 db with point_of_sale and account_accountant
- General Settings => Accounting => Check "Anglo-Saxon Accounting"
(debug mode)
- Choose or create a storable product P with category C
- Go to category C and set 'inventory valuation' to 'automated'
- Open a new POS session PS
- Sell product P with qty 1
- Pay
- New order
- Sell product P with qty -1 (refund)
- Pay
- Close session PS
- Validate session PS

Error "You are trying to reconcile some entries that are already
reconciled." will popup because the 0 sum AML is already marked as
reconciled. This can be fixed by avoiding asking reconciliation when the
amount_residual is 0.

opw-2177881

closes odoo/odoo#48696

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>