cmd/fetch: handle nil cask urls

These urls can be nil if there is an unsatisfied macos version
requirement. We check for false here because either the macos
requirement can be satisfied and return true or can not be
specified and return nil. If it's not specified, it means it
can run on any macos version.

The change in Cask::Download should provide better error messages
in Downloadable but honestly we're better off just checking for
the missing url higher up the call stack which is why I made
the changes in the fetch command. Either way it seemed like
a good idea while I'm here.

Library/Homebrew/cask/download.rb

@@ -24,6 +24,8 @@ module Cask
 
     sig { override.returns(T.nilable(::URL)) }
     def url
+      return if cask.url.nil?
+
       @url ||= ::URL.new(cask.url.to_s, cask.url.specs)
     end
 

Library/Homebrew/cmd/fetch.rb

@@ -167,6 +167,11 @@ module Homebrew
           SimulateSystem.with os: os, arch: arch do
             cask = Cask::CaskLoader.load(ref)
 
+            if cask.depends_on.macos&.satisfied? == false
+              opoo "#{cask.token}: #{cask.depends_on.macos.message(type: :cask)}"
+              next
+            end
+
             quarantine = args.quarantine?
             quarantine = true if quarantine.nil?