Cask::Audit: fix key not found: :latest error

`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.

Author: samford

Library/Homebrew/cask/audit.rb

@@ -755,9 +755,9 @@ def audit_livecheck_version
       latest_version = Homebrew::Livecheck.latest_version(
         cask,
         referenced_formula_or_cask: referenced_cask,
-      )&.fetch(:latest)
+      )&.fetch(:latest, nil)
 
-      return :auto_detected if cask.version.to_s == latest_version.to_s
+      return :auto_detected if latest_version && (cask.version.to_s == latest_version.to_s)
 
       add_error "Version '#{cask.version}' differs from '#{latest_version}' retrieved by livecheck."