[FIX] purchase_stock: Prevent picking update when product_qty isn't updated

_create_or_update_picking() is a time-consuming method.
On some Purchase, all the purchase.order.line may be written with 'product_qty' in 'values', even though it did not change.

OPW-2978569

closes odoo/odoo#114546

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