cmd/audit: fix type error in cask livecheck url audit

[apainintheneck]

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This got updated recently in 42a42c9 to split out the livecheck audits and as a result of that the type signature for the #validate_url_for_https_availablity method got updated.

This did not account for the possibility that the livecheck url was nil. I've added a nil check here since it makes no sense to try and validate a nil url and we have nil checks for the other two http availability audits already. Plus, the livecheck blocks are audited thoroughly already for syntax.

This should fix the audit error mentioned in #16438 (comment).

CC: @reitermarkus @bevanjkay

[MikeMcQuaid]

Thanks @apainintheneck!

[reitermarkus]

Thanks!

So when is the url nil exactly? When there is a livecheck without a url?

I'm confused how it happened for Homebrew/homebrew-cask#163945 given there is a livecheck with a url.

[bevanjkay]

It was on a monterey runner which has

livecheck do
  skip "Legacy version"
end