bump-cask-pr: ensure new_cask has a url

We sometimes see errors like "attempted to use a `Downloadable`
without a URL!" in the homebrew/cask autobump workflow log because
`bump-cask-pr` can simulate Linux even if a cask doesn't support it,
leading to this error. This is something that should be resolved in
the future once I finally wrap up my related work to detect OS/arch
requirements but this adds a simple guard to address this in the
interim time.

Library/Homebrew/dev-cmd/bump-cask-pr.rb

@@ -273,6 +273,8 @@ module Homebrew
               end
               languages.each do |language|
                 new_cask        = Cask::CaskLoader.load(tmp_contents)
+                next unless new_cask.url
+
                 new_cask.config = if language.blank?
                   tmp_cask.config
                 else