[FIX] purchase: origin field on "procurement.order"

The field origin is not required in "procurement.order" and
trying to concat a string with a boolean raises a Type Error.

opw:670307

addons/purchase/purchase.py

@@ -898,7 +898,10 @@ class ProcurementOrder(models.Model):
             elif not po.origin or procurement.origin not in po.origin.split(', '):
                 # Keep track of all procurements
                 if po.origin:
-                    po.write({'origin': po.origin + ', ' + procurement.origin})
+                    if procurement.origin:
+                        po.write({'origin': po.origin + ', ' + procurement.origin})
+                    else:
+                        po.write({'origin': po.origin})
                 else:
                     po.write({'origin': procurement.origin})
             res += po.ids