[FIX] purchase_requisition: currency updated on blanket order vendor change

Steps to reproduce:
- install contacts and purchase
- go to purchase > configuration > settings > activate "purchase agreements"
- go to general settings and activate multi-currency
- go to contacts > select a contact > edit > sale & purchase > change the currency
- go to purchase > purchase agreements > create
- change the agreement to a blanket order > add your modified contact as the vendor

Previous behavior:
the currency is not updated

Current behavior:
the currency is updated on vendor change

opw-2177431

closes odoo/odoo#43877

X-original-commit: 764135630949eb01cb2b3a4a19d6ec5b1bd39400
Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>

addons/purchase_requisition/models/purchase_requisition.py

@@ -75,6 +75,11 @@ class PurchaseRequisition(models.Model):
 
     @api.onchange('vendor_id')
     def _onchange_vendor(self):
+        if not self.vendor_id:
+            self.currency_id = self.env.company.currency_id.id
+        else:
+            self.currency_id = self.vendor_id.property_purchase_currency_id.id or self.env.company.currency_id.id
+
         requisitions = self.env['purchase.requisition'].search([
             ('vendor_id', '=', self.vendor_id.id),
             ('state', '=', 'ongoing'),