[FIX] account: avoid invoice name reset when we change the name manually

Before:
- Create a new sales journal
- Create a new first invoice
- Do not put an invoice date yet
- Change the name from 2023/000001 to e.g. 2023/000010
- Confirm the invoice
- The name will change back to 2023/000001 (which is not the wanted behaviour)

After:
We avoid the change back by adding a condition in the _compute_name, which
is triggered by the invoice_date being set while confirming.

X-original-commit: e31c5767
Part-of: odoo/odoo#111294