Cask: Ensure #discontinued? returns a boolean

The `Cask::DSL#discontinued?` method is expected to return a boolean
value (`true`/`false`) but it will return `nil` for any casks without
a `caveats` block. `nil` is still falsy but this commit ensures that
the method will properly return a boolean value in this scenario.

Library/Homebrew/cask/dsl.rb

@@ -281,7 +281,7 @@ module Cask
     end
 
     def discontinued?
-      @caveats&.discontinued?
+      @caveats&.discontinued? == true
     end
 
     # @api public