Format numeric values in XML templates using fixed-point notation

Closes #14948
(grafted from 6cb75123ee20de6cf90ff1bbd6189b64e4d1f739)

--HG--
branch : 8.0

modules/edocument_ubl/edocument.py

@@ -279,7 +279,7 @@ class Invoice(Model):
             elif self.invoice.currency == self.invoice.company.currency:
                 return line.debit - line.credit
             else:
-                return 0
+                return Decimal(0)
         return sum(map(balance, self.invoice.lines_to_pay))
 
     @classmethod