Fix exception if first order does not have billing address

uncloud_pay/models.py

@@ -327,7 +327,10 @@ class Bill(models.Model):
         orders = Order.objects.filter(bill=self)
         # The genrate_for method makes sure all the orders of a bill share the
         # same billing address. TODO: It would be nice to enforce that somehow...
-        return orders[0].billing_address
+        if orders:
+            return orders[0].billing_address
+        else:
+            return None
 
     # TODO: split this huuuge method!
     @staticmethod