Update node-loader.js to fix Windows compatibility with node 0.10.x

According to [the nodemon issue tracker](https://github.com/remy/nodemon/issues/150), there is an error thrown by path.join on Windows/Node 0.10.x which is : 

*src/node-loaders.js:28:45*
>TypeError: Arguments to path.join must be strings

The issue can be easily fixed by changing `path.join(dir, filename)` to `path.join(dir, filename || '')`.
I tested it and it actually works, the watcher doesn't throw an error anymore while watching templates on Windows/Node 0.10.x.