Allow empty hash on AMP invoices

[guggero]

This is part of the fix for https://github.com/lightninglabs/lightning-terminal/issues/632.

Pull Request Checklist

• [X] PR is opened against correct version branch.
• [X] Version compatibility matrix in the README and minimal required version in lnd_services.go are updated.
• [X] Update macaroon_recipes.go if your PR adds a new method that is called differently than the RPC method it invokes.

[ViktorT-11]

We need to add support for this i LiT as well. Just a heads-up to avoid complications, let's avoid merging this until we've built that support.