Default to an empty entity when the node cannot be found.

This allows the class_mapping functionality to run on nodes that don't
have (or need) their own node configuration file. So for example, if a
node isn't explicitly configured and you have a 'default' rule in your
class mapping, then it will run for all nodes, not just those that are
explicitly configured.

In my opinion this makes the class_mapping a much more powerful feature, as it allows you to fully configure a node based on regular expressions in the class mapping.
I have a feeling that maybe this should be configurable, so you can enable it and disable it, however I'm not certain how to go about doing that.

Any suggestions & comments would be nice. If this is a stupid idea and you know of another way to accomplish the same result, that would be awesome!