[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#42708

Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>

addons/account/wizard/account_validate_account_move.py

@@ -9,7 +9,7 @@ class ValidateAccountMove(models.TransientModel):
     def validate_move(self):
         context = dict(self._context or {})
         moves = self.env['account.move'].browse(context.get('active_ids'))
-        move_to_post = moves.filtered(lambda m: m.state == 'draft').sorted(lambda m: (m.date, m.ref, m.id))
+        move_to_post = moves.filtered(lambda m: m.state == 'draft').sorted(lambda m: (m.date, m.ref or '', m.id))
         if not move_to_post:
             raise UserError(_('There are no journal items in the draft state to post.'))
         move_to_post.post()