[FIX] account: Fix missing dependencies of _check_duplicate_supplier_reference

- Create a vendor bill for partner A with reference 'AAAAA'
- Create another vendor bill for partner A with the same reference:
=> Constraint is triggered.
- Create another vendor bill without partner with the same reference.
- Edit the vendor bill and set a partner:
=> Constraint not triggered.

closes odoo/odoo#40849

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>