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.
Merge pull request #72 from yaegashi/fix-asset-path-errors

Fix #asset_path errors against blank path requests