Fixes #13810 `rake routes` error when mount `Rails::Engine` with empty routes

This fixes an error which raises up when in a Rails app `config/routes.rb` is mounted a `Rails::Engine` with empty routes, f.e.:

##### Rails app `lib/my_engine.rb`

```ruby
class MyEngine < ::Rails::Engine ; end
MyEngine.routes.draw do ; end
```

##### Rails app `config/initializers/my_engine.rb`

```ruby
require 'my_engine'
```

##### Rails app `config/routes.rb`

```ruby
Rails.application.routes.draw do
  mount MyEngine => 'my-engine'
end
```

##### Rails app `rake routes --trace`

```
$ rake routes --trace
** Invoke routes (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute routes
rake aborted!
comparison of Fixnum with nil failed
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:183:in `each'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:183:in `max'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:183:in `map'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:183:in `draw_section'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:163:in `section'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:105:in `block in format'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:103:in `each'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/actionpack-4.1.0.beta1/lib/action_dispatch/routing/inspector.rb:103:in `format'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/railties-4.1.0.beta1/lib/rails/tasks/routes.rake:6:in `block in <top (required)>'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:236:in `call'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:236:in `block in execute'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:231:in `each'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:231:in `execute'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:175:in `block in invoke_with_call_chain'
/.rbenv/versions/2.1.0/lib/ruby/2.1.0/monitor.rb:211:in `mon_synchronize'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:168:in `invoke_with_call_chain'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/task.rb:161:in `invoke'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:149:in `invoke_task'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `block (2 levels) in top_level'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `each'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:106:in `block in top_level'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:115:in `run_with_threads'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:100:in `top_level'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:78:in `block in run'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:165:in `standard_exception_handling'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/lib/rake/application.rb:75:in `run'
/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/rake-10.1.1/bin/rake:33:in `<top (required)>'
/.rbenv/versions/2.1.0/bin/rake:19:in `load'
/.rbenv/versions/2.1.0/bin/rake:19:in `<main>'
/.rbenv/versions/2.1.0/bin/ruby_executable_hooks:15:in `eval'
/.rbenv/versions/2.1.0/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => routes
```