[FIX] account: multi-post order compare bool and str

The `ref` field on `account.move` is not required and can yield a falsy
value. This produces a TypeError while doing the comparison in sorted().

closes #42434

closes odoo/odoo#43674

X-original-commit: 3bb4c0c9
Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>