Cost on product history may be NULL

The product cost history query table may return None for a cost price. This is because the query is based on the stock move for which the cost price is not required at the database schema but only by validation on specific state. As this requirement may evolve over time (or maybe wrong due to bug), we can not be sure that the history is always not null. I think we should not use stock move with a null cost price. Otherwise get_cost_price_at may raise an Exception for rounding None as price.