[FIX] account: use current date as fallback to convert currency

Description of the issue/feature this PR addresses: Fixes #76177

Current behavior before PR:
Before this commit, There will be tracback on adding new
Journal Item if there is not date on Journal Entry.

Desired behavior after PR is merged:
Now we use current date to as fallback to convert currency.

closes odoo/odoo#76178

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>