Fixes related to handling of empty types (#600)

PR #577 tries to eliminate empty `struct` types by replacing them with a `LegalType::tuple` with zero elements, but this seems to run into problems in some cases, where we end up trying to match up `::none` values with empty `::tuple`s.

An alternative way to handle this is to never create empty `LegalType::tuple`s (and the same for `LegalVal::tuple`), and instead create `LegalType::none` and `LegalVal::none`. PR #577 avoided this because there were various cases in the legalization logic that didn't robustly handle `LegalType::Flavor::none`.

This PR thus includes two main changes:

1. Construct a `::none` type when we have an empty `struct` type.

2. Survery all places that handle the `::tuple` case and extend them to handle the `::none` case if it was missing.

This fixes an issue filed in Falcor's internal GitLab as number 424.

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