Export of invoice, Magento may return an empty ID, provoking a dup constraint error

[Guewen Baconnier @ Camptocamp]

When we create an invoice on Magento, it may happen that instead of returning an ID, it returns Nothing.

In that case, we'll bind the magento.account.invoice with 'None' (as str! the binder must not allow that). And as soon as 2 bindings have the same issue, both will be bound with 'None', raising a duplicate constraint error.