[FIX] purchase_stock: relax the location constrains

The constrains on orderpoint location being related to the warehouse view
location was too restrictive especially in a complex subcontracting flow
with dropship.

This commit change the constrains to only be triggered if the two
location to be compared have both a warehouse.

closes odoo/odoo#79126

Signed-off-by: Arnold Moyaux <arm@odoo.com>