[FIX] account: fallback journal

Commit 6b14ceb2 introduced a smarter way to find the default
journal based on the currency. However, it missed the fallback if no
journal is found.

opw-2009934
opw-2024204
opw-2011321

closes odoo/odoo#34263

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>