[FIX] mrp_subcontracting: if picking type is empty, no picking as to be created at PO confirmation

[jbaudoux]

When the picking type is empty, it raises an exception as it tries to create a picking without picking type and this field is a required in a picking.

In my scenario, all raw materials are provided directly at the subcontractor and the finished product is send trough transit operation to next subcontractor or though delivery order to end-customer.
There is no main warehouse and so no picking in/out to the main warehouse. All is manufactured though several subcontractors.
The MO generates a PO to the subcontractor with a service line (cost of the manufacturing)
This scenario works properly with provided odoomrp modules except PO cannot be confirmed without this fix.