fix: warning: instance variable @controller not initialized

[KapilSachdev]

Fixes #39937

Summary

As described in the #39937, @controller is not always defined while testing routes, and thus the line

rails/actionpack/lib/action_dispatch/testing/assertions/routing.rb

Line 202 in 23a9e29

request = ActionController::TestRequest.create @controller.class

might raise warning: instance variable @controller not initialized in some instances.

This can be handled by checking defined?(controller) before accessing the instance variable.

controller = @controller if defined?(@controller)
request = ActionController::TestRequest.create controller&.class

Thanks to @ioquatix for the report (and solution as well :) )