Can not handle empty struct variants

[tolak]

Under the current implementation, we iterate the variants returned in metadata, but sometimes, the variants will not be iteraterable, e.g. it will be an empty object {}. For example, if we have an empty Error defined in ink contract:

    #[derive(Encode, Decode, Debug, PartialEq, Eq)]
    #[cfg_attr(feature = "std", derive(scale_info::TypeInfo))]
    pub enum Error {
        // Empty, will lead to empty variants in metadata
    }

This will make the program crash, we should take care of it

[l00k]

Thx, Fixed in v0.0.29