[FIX] account: fix "account.bank.statement.cashbox" export with data update

- Go to Point of Sale
- Switch to View list
- Select a POS and click on Action\Export
- On Export wizard:
  * check "I want to update data (import-compatible export)" option
  * expand "Default Balance" section
A KeyError is triggered while trying to access undefined field referred by "_rec_name"
of "account.bank.statement.cashbox".

opw-2309064

closes odoo/odoo#55480

X-original-commit: 7e3143b9
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>
Signed-off-by: Anh Thao PHAM <kitan191@users.noreply.github.com>