this fix a error throwed when the main scss file is empty

This PR fixes the following error when run `gulp`:

```
$ gulp
Using gulpfile ~/development/xteam/remember/Gulpfile.js
Starting 'styles'...
Starting 'scripts'...
Starting 'watch'...
Finished 'watch' after 6.68 ms
Starting 'nodemon'...
Finished 'nodemon' after 966 μs
gulp: ../node_modules/nan/nan.h:1725: bool _NanGetExternalParts(v8::Handle<v8::Value>, const char**, size_t*): Assertion `val->IsString()' failed.
Aborted (core dumped)
```