Handle preprocessor_path == ""

When using the asset pipepline, the font-url helper need to receive the name of the font width a slash in the beginning to properly compile assets.

This modifications adds a new ternary to handle the case where preprocessor_path is "" and don't put a slash with the name.
Merge branch 'patch-1' of https://github.com/wlepinski/fontcustom into wlepinski-patch-1
Expands double ternary