[FIX] stock: merge fields of several moves

If `_merge_moves_fields` is called with a recordset of several SM, and
if there are more than one `self.picking_id`, the method will trigger a
`ValueError` (Expected singleton)

OPW-2861605

closes odoo/odoo#99916

Signed-off-by: William Henrotin (whe) <whe@odoo.com>