[FIX] stock: update description_picking with picking type

This commit updates the description_picking on stock moves once the
picking type is changed. This is needed to merge stock moves at
confirmation. The description_picking field chosen on the merged move is
the 'minimum one'. Empty string will always be chosen that way, so we
lose the information.

closes odoo/odoo#60711

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>