Null should yield empty string when resolving variable

This is related to #236. Am upvoting for showing empty string when resolving `null` object.

This is more similar to engines that swig is based on (Jinja/Django both resolves `None` as empty strings) and most of the time is more desired (I guess) than opaque `[Object object]`.

While writing tests was pretty straightforward, have to admit I am not sure about the implementation.

Let me know what you think.