Fix for empty default variables used as paths.
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
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp