cask: handle nil URLs

[Bo98]

This generally shouldn't happen but it's probably acceptable for casks under the minimum macOS.

There are a few cases I see this happen but for above the minimum macOS. The fix here still covers them, but I'll open a change to readall to catch that there as I don't think the cask should be called valid in those scenarios.

[BrewTestBot]

Review period skipped due to critical label.

[Bo98]

(This fixes formulae.brew.sh generation)

[MikeMcQuaid]

Thanks @Bo98!

Should probably use brew generate-*-api commands in Homebrew/homebrew-core and Homebrew/homebrew-cask jobs as required steps to avoid breaking API generation with a cask merge.

[Bo98]

It was actually a brew change that caused it.

For homebrew-core/cask: that makes sense. Though I'm of the belief that brew readall should cover it and if it doesn't then we've got poor validation (hence the change I proposed to that).

[MikeMcQuaid]

It was actually a brew change that caused it.

@Bo98 same deal then, I guess: we should make this commands part of the CI workflow so we can't break them.

Though I'm of the belief that brew readall should cover it and if it doesn't then we've got poor validation (hence the change I proposed to that).

It should but I think it's not safe for us to rely on that. There's many other ways this can get broken that readall might not pick up.