[UPD] Update stock_picking_import_serial_number.pot

You can't use a static assignation in kw args, and less the empty
string, which is assigned as is on the char fields (and being different
from NULL values).

Don't pass `default_code` in vals if no reference is provided, for
letting other modules to work properly in integration tests like
`product_code_mandatory` + `product_sequence`.