Fix Custom Functions PHP code for Shelf Life functionality

We currently have a Best Before Basket field, and we need some additional checks made before displaying it at checkout. At the moment if the basket is digital products only (gift cards) then the Delivery Date field will not show (as there is no physical delivery). In this instance we would also need the 'Best Before Basket' date to be hidden.
Along with this, we are now allowing non-selected/blank Delivery Date, so if there is no customer entry for Delivery Date then this needs to be accounted for too. If no Delivery Date is used (i.e ASAP) then we need the 'Best Before Basket Date' to be calculated as the Shelf Life + 1 day (for ASAP delivery)