[FIX] purchase: do not set currency_id to false

- currently, cuurency_id has default value as currency of user's company id. but, when there is no partner set then 'onchange_partner_id' will trigger and set currency_id to false.
- so, set user's currency_id instead of false if no partner set.

Related to Issue: 1948645

closes odoo/odoo#32415

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

addons/purchase/models/purchase.py

@@ -192,7 +192,7 @@ class PurchaseOrder(models.Model):
         if not self.partner_id:
             self.fiscal_position_id = False
             self.payment_term_id = False
-            self.currency_id = False
+            self.currency_id = self.env.user.company_id.currency_id.id
         else:
             self.fiscal_position_id = self.env['account.fiscal.position'].with_context(company_id=self.company_id.id).get_fiscal_position(self.partner_id.id)
             self.payment_term_id = self.partner_id.property_supplier_payment_term_id.id