Incoterm on supplier invoice is empty even if filled on purchase order

As supplier shipment is created manually, there's no default incoterm and most of the time people doing the shipment doesn't know about incoterm, so field stay empty.

To display incoterm on the supplier invoice, the property incoterm name check first incoterm on shipment (which is empty) and don't go on so incoterm on supplier invoice is empty even if it was filled on the purchase order. (https://foss.heptapod.net/tryton/tryton/-/blob/314c49ddb2043362045493eac49d998cf39f2429/modules/incoterm/account.py#L24)

To avoid this, if would be better to check first if purchase to fill the incoterm otherwise stock moves.