The smithing recipe types in vanilla have three input fields: template, base, and addition. In vanilla, each of these fields can be set to an empty list to have no item required in the corresponding slot.

In Bukkit's SmithingRecipe, each field is marked as @NotNull and using a datapack that provides a recipe with one of these empty lists will cause the datapack to fail to validate.

A datapack has been attached below. It should allow the place to craft an Iron Block into a Gold Block as seen below, but on Spigot/derivative server software it throws an error.