Keep last cost price when quantity is zero
Since rev f3fcb65989bf, we recompute the cost price on moves but when the quantity stored in zero, we set the "average" cost price to zero.
But this is not the same behavior as when move is done where we keep last cost price (from Move._compute_product_cost_price).
This was pointed in https://codereview.tryton.org/327491003/diff/315661002/modules/stock/move.py#newcode498