Fix block default rendering in extends templates

[seedship]

There is a bug in inja where given a template:

skeleton:

{% block data %}some data{% endblock %}

using_skeleton:

{% extends "skeleton" %}

If the subconfig does not define anything, it will be generated as empty instead of using the default value. You would expect inja processing skeleton to generate:

some data

But instead it is just an empty file. This fixes it