Check if result is null

This may be the case if files are ignored. I do
not know what the other cases are, and if they
should be handled differently, but at least they
don't cause error like this.

See babel#265

index.js

@@ -28,7 +28,7 @@ Babelify.prototype._flush = function (callback) {
       this.emit("error", err);
     } else {
       this.emit("babelify", result, this._filename);
-      var code = result.code;
+      var code = result !== null ? result.code : this._data;
       this.push(code);
       callback();
     }