Ignore failed signature check if no signature is present.

Library/Homebrew/cask/audit.rb

@@ -581,7 +581,13 @@ module Cask
           next unless path.exist?
 
           result = system_command("codesign", args: ["--verify", path], print_stderr: false)
-          add_warning result.merged_output unless result.success?
+
+          next if result.success?
+
+          # Only fail if signature is wrong, not when no signature is present at all.
+          next result.stderr.include?("not signed at all")
+
+          add_warning "Signature verification failed: #{result.merged_output}"
         end
       end
     end