Fix warnings and fallback to indexes as tags when labels are missing

[KodrAus]

This PR cleans up some new dead code lints and attempts to fall back to streaming a tag as its index if it has one but is missing a label.

[KodrAus]

I've made the treatment of indexes consistent, so if there's an index but no label then we'll attempt to use it, either as the value, or as the key of the variant. If there's no label and no index then we'll treat the variant as null, so far tags and empty enums that;ll produce null, and for variants it'll be as if they're untagged.