When Magento returns no ID when we create an invoice...

...try to get the invoice's ID otherwise, just consider the job as done, but do not bind with a None ID

Bug report: https://code.launchpad.net/bugs/1337778
Original MP on Launchpad: https://code.launchpad.net/~camptocamp/openerp-connector-magento/7.0-export-invoice-no-id-1337778/+merge/225628