Fix brew outdated --greedy crash on installed casks without a URL

[p-linnane]

After installed cask metadata migration was enabled for all users, brew outdated --greedy began failing with Error: attempted to use a Downloadable without a URL! on :latest casks whose minimal installed JSON omits url (reported in https://github.com/orgs/Homebrew/discussions/6961).

CaskStructGenerator built url_args as [hash["url"].to_s], so an absent url became [""]. That one-element array is not blank, so it survived compact_blank and reached url("") in the loader. Since "" is truthy, the DSL wrapped it in a real URL instead of leaving the url unset, making checksumable? true and sending greedy outdated detection into the downloader.

Calling compact_blank on the array drops the blank element, so an absent url yields no arguments, cask.url is nil, and checksumable? returns false. Such :latest casks are still reported outdated under --greedy, now without touching the downloader. This matches the internal API round-trip, which already strips blank url_args on serialization.

The regression test reconstructs URL-less installed :latest metadata with an existing LATEST_DOWNLOAD_SHA256 and asserts the greedy check returns instead of raising.

---

• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

---

• AI was used to generate or assist with generating this PR.

Claude Code (Opus 4.8) diagnosed the empty-URL path and wrote the fix and regression test; I reviewed the diff and verified with brew lgtm plus targeted cask/cask_loader/from_api_loader, api/cask_struct, and api/cask/cask_struct_generator specs.

---

[Copilot]

Pull request overview

This PR fixes a regression where brew outdated --greedy could crash for installed :latest casks whose migrated installed JSON metadata omits url, by ensuring API cask struct generation does not produce a blank url("") stanza.

Changes:

• Drop blank url values when generating url_args for cask structs, so URL-less installed metadata yields cask.url == nil.
• Add a regression spec that reconstructs URL-less installed :latest metadata with an existing LATEST_DOWNLOAD_SHA256 and asserts greedy outdated detection returns without raising.

Reviewed changes

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

File	Description
Library/Homebrew/api/cask/cask_struct_generator.rb	Ensures missing url does not become [""], preventing url("") from being set in the loader path.
Library/Homebrew/test/cask/cask_loader/from_api_loader_spec.rb	Adds coverage for the URL-less installed-metadata scenario that previously triggered the downloader crash under --greedy.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MikeMcQuaid]

Thanks!