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:

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New Features:

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Bugfixes:

• This pull request breaks the existing API
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