Add unit_price in moves even when it is not required
1.Create a return purchase. 2.Confirm the purchase. 3.This action create a return shipment (stock.shipment.in.return). 4.Open this return shipment. 5.We can have a "from_location" with the type "view", in this case, the unit_price will be empty. (For example, if we have all the return location inside a specific view location) 6.If we try to assign this shipment, tryton will return the UserError "A value is required for field "Unit Price" in "Stock Move". 7.Then if we try to change the "from_location" to one where the type is "Internal" the unit_price field will still be empty.
I think the field "unit_price" need to be always filled, even when it is not necessary. This means we need to delete the if condition here: https://hg.tryton.org/modules/purchase/file/tip/purchase.py#l1679