[FIX] spreadsheet: get currency rate without date

Commit f693d9b5 broke ODOO.CURRENCY.RATE when no date is provided.
The text 'Invalid date' is sent to the server, which obvious can't
parse it to a date.

opw-3498115

closes odoo/odoo#136148

Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>