[FIX] l10n_be_invoice_bba: Duplicating an existing in_invoice with BBA structured message

Steps to reproduce the bug:

- Create a partner P with communication type set to BBA

- Create an in_invoice I for P with a BBA structured communication

- Validate I and try to duplicate it

Bug:

A UserError was raised "Empty BBA Structured Communication!"

opw:1883982

closes odoo/odoo#27331