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

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.

Library/Homebrew/cask/audit.rb

@@ -806,10 +806,11 @@ def audit_url_https_availability
     def audit_livecheck_https_availability
       return unless online?
       return unless cask.livecheckable?
-      return if cask.livecheck.url.is_a?(Symbol)
+      return unless (url = cask.livecheck.url)
+      return if url.is_a?(Symbol)
 
       validate_url_for_https_availability(
-        cask.livecheck.url, "livecheck URL",
+        url, "livecheck URL",
         check_content: true
       )
     end