[FIX] account_accountant: fix multiple liquidity lines

When multiple liquidity lines exist on an account.move (occuring before
14.0), the bank reconciliation widget crashes. Taking the sum of the
`amount` and `amount_currency` solves it.

opw-3202392

closes odoo/odoo#114126

Signed-off-by: Laurent Smet <las@odoo.com>