Fix for empty default variables used as paths.

With r1637 default varaibles used as paths are now being relativized.
However, if the default variable used as a path is empty, gyp throws an error.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/15968005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1638 78cadc50-ecff-11dd-a971-7dbc132099af