cask/audit: fix nil crash in min_os check

[krehel]

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I identified the crash when working on the lm-studio cask, debugged with Opus 4.8 and tested both by Opus and by hand. Tests run using brew lgtm --online

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audit_min_os compares depends_on_min_os and cask_min_os against HOMEBREW_MACOS_OLDEST_ALLOWED without nil guards. Both are nil for a cask with no depends_on macos: and no versioned on_system block, so when the artifact's minimum macOS exceeds the floor the audit raises undefined method '>' for nil (reported as "exception while auditing ") instead of the intended "cask declared no minimum macOS version" error.

macOS-only casks declare a minimum OS requirement using the depends_on macos: syntax, so the nil path was only exposed by cross-platform casks that scope macOS artifacts in an on_macos block and declare no macOS version.

This guards the nil cases so the audit doesn't crash and add a regression test for an app requiring a newer macOS with no declared minimum.

[Copilot]

Pull request overview

Fixes a crash in Cask::Audit’s minimum macOS version audit when a cask has neither a depends_on macos: stanza nor a versioned on_system minimum, ensuring the audit reports the intended error instead of raising an exception.

Changes:

• Add nil guards around depends_on_min_os/cask_min_os comparisons and logging in audit_min_os.
• Add a regression spec covering a macOS-scoped artifact with no declared minimum macOS version, where the artifact requires a newer macOS.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/cask/audit.rb	Adds nil-safe handling in audit_min_os to prevent nil > ... crashes and keep the audit reporting a proper error.
Library/Homebrew/test/cask/audit_spec.rb	Adds a regression test for the nil-minimum-OS path to ensure the audit errors cleanly instead of crashing.

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[MikeMcQuaid]

Thanks!