decide how to to support empty baseTemplate

[ghost]

There are two easy ways to fix this todo:

TODO: only extend base template if base template string is not empty

we could do something like this:

{% extends baseTemplate ? baseTemplate : "empty.html.twig" %}

OR

{% extends [baseTemplate, 'empty.html.twig'] %}

as per this http://twig.sensiolabs.org/doc/tags/extends.html#dynamic-inheritance

the 2nd might be more exciting if we want multiple fallbacks

[c33s]

would be ok for me

[vworldat]

:+1:

[ghost]

fixed this one too in #35