Return a blank JSON response for a missing .js file to prevent Rails CSRF errors

The default 404 handler would return the Content-Type format based on the
given format extension. This would cause the Rails CSRF protection to flag an
error, since the .js extension gets mapped to text/javascript format.

Closes #40771

app/controllers/application_controller.rb

@@ -147,6 +147,8 @@ class ApplicationController < ActionController::Base
       format.html do
         render file: Rails.root.join("public", "404"), layout: false, status: "404"
       end
+      # Prevent the Rails CSRF protector from thinking a missing .js file is a JavaScript file
+      format.js { render json: '', status: :not_found, content_type: 'application/json' }
       format.any { head :not_found }
     end
   end