Avoid nil cask versions during installation

[MikeMcQuaid]

• Platform-specific casks can have no version when evaluated on an unsupported operating system.
• Let upgrade detection skip latest? in that case so installation reaches normal platform validation instead of raising NoMethodError.
• Cover a macOS-only cask evaluated on Linux.

Fixes #23498

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OpenAI Codex 5.6 Sol xhigh with local review and testing.

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[Copilot]

Pull request overview

This pull request fixes a crash in Homebrew’s cask upgrade-detection path during brew install --cask when a cask evaluates to having no version on the current OS (e.g., a macOS-only cask evaluated on Linux). It ensures installation proceeds to normal platform validation instead of raising a NoMethodError.

Changes:

• Guarded latest? checks in Cask::Upgrade.outdated_casks when cask.version can be nil.
• Added an RSpec regression test covering a macOS-only version stanza evaluated under Linux simulation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/cask/upgrade.rb	Avoids calling latest? on a nil version during outdated-cask selection.
Library/Homebrew/test/cask/upgrade_spec.rb	Adds coverage for the nil-version-on-unsupported-OS scenario.

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