Add warning for missing product weight in shipping estimates

When generating shipping estimates via an integration, the current system does not warn the user if one of the items in a sales order has a weight of 0, as long as something else in the order has a weight.  This means that if a heavy item is somehow missing weight data, the shipping estimate will be underestimated and no warning will be provided. I confirmed this behaviour with Cin7 Support.

It would be a huge help if the system could trigger an alert or warning whenever an item in a sales order is missing these details. Having this check in place would make it much easier for our team to spot and fix incomplete product information. Otherwise the shipping estimates will be inaccurate.