Fix #asset_path errors against blank path requests

It raised ```#<TypeError: no implicit conversion of nil into String>``` from ```Pathname#new``` when passed a blank ```pathname```.  It should return an empty string without error in this case, following the behavior of [```ActionView::Helpers::AssetUrlHelper#asset_path```](https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/asset_url_helper.rb#L120).

This is a revised pull request of #71.