[FIX] account: Fix corner cases of https://github.com/odoo/odoo/pull/70303

Original issue:
Create a payment like follow:

Payment type: receive money
Partner type: customer
Destination account: whatever receivable
Amount: define an amount (i.e. 90.00)
On Save, open the draft jounral entry created.
Lower the liquidity or counterpart amount and add a writeoff
Go back to the payment, change the amount and save.
The journal entry writeoff will swap from debit to credit at every
change made to the amount. This occur also when changing currency on the
payment

opw-2475223

Otherwise, this fix is not covering all cases:
inbound with write-off in debit
inbound with write-off in credit
outbound with write-off in debit
outbound with write-off in credit

closes odoo/odoo#70807

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>