[FIX] account: fix TypeError for currency in journal items

If applied, this commit will solve the issue of unsupported operand type in
journal item(s) when currency is not set.

Steps to produce:
- Accounting > Journal Entries > Create new entry.
- Add a line in Journal Items and remove the currency from that line.
- Save > Error will be generated.

Fix this issue by calling the compute of the currency when the currency is not
available.

sentry - 4069681536

closes odoo/odoo#122462

Signed-off-by: Florian Gilbert (flg) <flg@odoo.com>