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Cask::Audit.audit_livecheck_version can raise a key not found: :latest error when a hash from livecheck's latest_version method doesn't have a :latest value. This error means that livecheck was unable to identify the latest upstream version but it can only be understood if the reader knows how this audit is implemented (and it may also depend on knowing the structure of livecheck's latest_version hash). Without that knowledge, the error doesn't make it clear which audit is failing and why.

This addresses the issue by using nil as the default value for this fetch call and accounting for a nil latest_version value. This allows the audit to surface the usual "Version '1.2.3' differs from '' retrieved by livecheck" failure, which makes it more clear that livecheck isn't returning a version.