don't fail on finders without storages attribute

I'm using https://github.com/ifanrx/django-dajaxice and after upgrading to 1.3.23 not all css files are loaded with css tag. It turns out that pipeline was failing on ``finder.storages.values()`` call since finder in dajaxice doesn't have storages attribute. It was working with <1.3.22 since there was return before it reached dajaxice finder.