[FIX] purchase: prevent unnecesary change of currency

Steps to reproduce:
- put the currency of the bill to eur
- change the partner with a partner with no purchase currency set

Issue:
The bill is re-set to usd

Note:
addendum to https://github.com/odoo/odoo/pull/116852



opw-3233527

closes odoo/odoo#118350

Signed-off-by: Cedric Snauwaert <csn@odoo.com>