Do not fail when testing if account is set on invoice line for project

If there is no product defined on the project, the call to on_change_product
for the invoice line does not set the account field.

Closes #13111
(grafted from 01992016f36efdf8576a2453a084ba642e43d74f)

--HG--
branch : 7.0

modules/project_invoice/project.py

@@ -592,7 +592,7 @@ class Work(Effort, Progress, Timesheet, metaclass=PoolMeta):
         invoice_line.unit = key['unit']
         invoice_line.product = product
         invoice_line.on_change_product()
-        if not invoice_line.account:
+        if not getattr(invoice_line, 'account', None):
             if invoice_line.product:
                 raise InvoicingError(
                     gettext(