Don't generate assets' initializer in `app:update` task if sprockets is skipped

Execute `rails new myapp -S` and then upgrade the app by using the `app:update` task, `bin/rails c` results in `NoMethodError`.

```
$ bin/rails app:update

$ bin/rails c
Traceback (most recent call last):
    44: from bin/rails:4:in `<main>'
(snip)
     1: from /Users/tanimichi.tsukuru/ghq/github.com/moneyforward/moneyplus/config/initializers/assets.rb:4:in `<top (required)>'
/Users/tanimichi.tsukuru/ghq/github.com/moneyforward/moneyplus/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/railtie/configuration.rb:97:in `method_missing': undefined method `assets' for #<Rails::Application::Configuration:0x00007fcb8d3697e0> (NoMethodError)
Did you mean?  asset_host
```

1 files changed, 13 insertions, 0 deletions

diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index fa7bcfd6d6..938d545219 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -296,6 +296,19 @@ class AppGeneratorTest < Rails::Generators::TestCase
     end
   end
 
+  def test_app_update_does_not_generate_assets_initializer_when_skip_sprockets_is_given
+    app_root = File.join(destination_root, "myapp")
+    run_generator [app_root, "--skip-sprockets"]
+
+    stub_rails_application(app_root) do
+      generator = Rails::Generators::AppGenerator.new ["rails"], { update: true, skip_sprockets: true }, { destination_root: app_root, shell: @shell }
+      generator.send(:app_const)
+      quietly { generator.send(:update_config_files) }
+
+      assert_no_file "#{app_root}/config/initializers/assets.rb"
+    end
+  end
+
   def test_app_update_does_not_generate_action_cable_contents_when_skip_action_cable_is_given
     app_root = File.join(destination_root, "myapp")
     run_generator [app_root, "--skip-action-cable"]