[FIX] stock: _assign_picking

Commit[1] tried to improve the usability of the pick + pack + ship + a
fixed procurement group scenario by erasing the origin value on the
groupped ship that refered wrongly to only the first downstream document
(and in fact refered to multiple).

We forgot the scenario into which we add a sale order line. In this
scenario, we also go through `_assign_picking` and, if an existing open
picking is found, we also wiped its partner and origin value, which was
indeed wrong.

We fix this scenario by being more precise on the erasing condition.

[1] 4a5f0765