[FIX] purchase: prevent traceback onchange partner

Steps to reproduce:
In an account move, if the partner_id is changed to one that does not have a value assigned in the property_purchase_currency_id field and with a value in the context for default_currency_id,
when passing through the _onchange_partner_id function of the purchase module,

Cause:
the variable currency_id will take the value in the context as second option causing an error when trying to get the value in currency_id.id because currency_id will be an integer and not a record.

issue-121232

closes odoo/odoo#121620

Signed-off-by: Yolann Sabaux (yosa) <yosa@odoo.com>