Dummy app generator: Only configure app/assets/javascripts if present

Newer versions of Rails generators do not generate the
`app/assets/javascripts` directory. Let's not add it to the Sprockets
config if it's not there.

(cherry picked from commit ca1af55)

Author: mamhoff

core/lib/generators/spree/dummy/dummy_generator.rb

@@ -54,6 +54,7 @@ def generate_test_dummy
     def test_dummy_config
       @lib_name = options[:lib_name]
       @database = options[:database]
+      @has_javascripts = Dir.exist?("#{dummy_path}/app/assets/javascripts")
 
       template "rails/database.yml", "#{dummy_path}/config/database.yml", force: true
       template "rails/storage.yml", "#{dummy_path}/config/storage/test.yml", force: true

core/lib/generators/spree/dummy/templates/rails/manifest.js renamed to core/lib/generators/spree/dummy/templates/rails/manifest.js.tt

@@ -1,3 +1,5 @@
 //= link_tree ../images
+<% if @has_javascripts %>
 //= link_directory ../javascripts .js
+<% end %>
 //= link_directory ../stylesheets .css