fix: return an error for empty silent payment payloads

[reez]

Description

Generic Bech32m permits an empty payload, but SilentPaymentCode::try_from assumes the decoded payload contains a version and panics when it does not. This returns WrongPayloadLength instead and adds regression coverage.

Notes to the reviewers

Changelog notice

Return an error instead of panicking when parsing a silent payment code with an empty payload.

Checklists

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran just p (fmt, clippy and test) before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[nymius]

Can you rebase on master to get the mutants fix?

[reez]

Can you rebase on master to get the mutants fix?

Definitely.

This is rebased now.

[nymius]

ACK 582ff44