Add to_s for nil path to continue support for carrierwave_direct

URI.rb was giving "no method gsub on Nil" when using with carrierwave_direct's recommended way of doing browser uploads directly to S3. In that instance you don't yet have a path (there's no file), but you use `SomethingUploader.direct_fog_url` as the destination to POST to. 

This just adds a "to_s" because path is nil.

I'm submitting this without being able to run tests locally - apparently there's a Travis CI server which can give me a result if I submit this as-is.