Allow Deleting of List if they only have 1 list left / Make sure the Products stay within the array

I found some basic issues with the bronto api

1. If a user is only on one list, the request would not send null to force them to be removed 
2. Products wouldn't be saved within the array since there was no public variable to store them in

I don't know how I feel about this one. If you accidentally update an order with an empty product array, it'll remove all products associated with that order (null will simply not update the products array). That said, there's something to say about initializing the products array if it is null as an array when you are adding products. Remove this line, and do an empty check on `$this->products` in the addProduct method, and I'll take it.