[FIX] stock: create backorder for all pickings in a batch

Create two pickings, add them to a batch.
Set a quantity less than the order to done on each.
Confirm and set to done. A wizard asks if a backorder should be created.
If confirmed, the first picking is done with a backorder but not the second.

Since the method is in api.multi, we can simply get all pickings.

opw 2004727

closes odoo/odoo#33902

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>

addons/stock/models/stock_move.py

@@ -1137,7 +1137,7 @@ class StockMove(models.Model):
                 .filtered(lambda p: p.quant_ids and len(p.quant_ids) > 1):
             if len(result_package.quant_ids.mapped('location_id')) > 1:
                 raise UserError(_('You cannot move the same package content more than once in the same transfer or split the same package into two location.'))
-        picking = moves_todo and moves_todo[0].picking_id or False
+        picking = moves_todo.mapped('picking_id')
         moves_todo.write({'state': 'done', 'date': fields.Datetime.now()})
         moves_todo.mapped('move_dest_ids')._action_assign()