light client guide: existence/non-existence proofs

[colin-axner]

Summary

Explain how existence and non-existence proofs are used within core IBC and how they might be implemented

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[colin-axner]

@AdityaSripal can you comment if there is anything specific that should be included in this documentation?

[AdityaSripal]

We should include how to "mock" non-existence proofs if the counterparty does not have ability to prove absence.

Since the verification method is designed to give complete control to client implementations, clients can support chains that do not provide absence proofs by verifying the existence of a non-empty sentinel ABSENCE value. Thus in these special cases, the proof provided will be an ICS-23 Existence proof, and the client will verify that the ABSENCE value is stored under the given path for the given height.