An empty Sass file should produce an empty CSS file

Currently if the input file is blank we bail out early, returning
the input file object. This causes gulp to simply copy the input
Sass file. We instead need to rewrite the file extension to .css.
Fixes dlmanning#352

index.js

@@ -29,6 +29,7 @@ var gulpSass = function gulpSass(options, sync) {
       return cb();
     }
     if (!file.contents.length) {
+      file.path = gutil.replaceExtension(file.path, '.css');
       return cb(null, file);
     }