Lot Expiration is checked when life state is set to None

Configuration

1. Create product. Select "Shelf Life Time State" to optional.
2. Create a lot and select a date in the past (before today) in "Shelf Life Expiration Date" field.
3. Create a shipment, production... and select in the moves, product and lot.

Scenario A

• When do a move, a validation raise in check_expiration_dates() because to_check_expiration() method return True [1]. It's ok.

Scenario B

• Go to the product, and change "Shelf Life Time State" to "none".
• Refresh lot form view. "Shelf Life Expiration Date" is now invisible.
• In case try do the move, continue raise the error from Scenario A.

Proposal

Check value option in shelf_life_state product in to_check_expiration() method.

    @property
    def to_check_expiration(self):
        if (self.lot
+                and self.product.shelf_life_state != 'none'
                and self.lot.shelf_life_expiration_date
                and self.effective_date > self.lot.shelf_life_expiration_date):
            return True
        return False

[1] https://github.com/tryton/tryton/blob/9096dda5b460426f579c04c49d00e84809bf9ec0/modules/stock_lot_sled/stock.py#L184