Fixing check for whether Rails app is using asset pipeline.

This modification was necessary for a rails app that has `config.assets.enabled = false` in config/application.rb.

Note that there's another way in which the revised check is not strictly equivalent to the earlier one: the revised version will return true if anything in the chain returns `nil`. I think this is fine, since Rails.application.assets should be truthy by default, afaik, but I could be wrong.