Allow empty templates.

[tancred]

No description provided.

[tancred]

My rationale behind this change is that I have templates supplied by users and a mechanism for including templates in other templates. Allowing for empty templates makes template management and error handling a lot easier.

Not sure if this is the best of even correct place for handling empty templates. For example, the changed assertions in TDTemplateEngine and TDTextNode could perhaps be removed altogether? And perhaps there's a way to return an "empty string node" a lot earlier?

Comments are most welcome. And thanks for a great library!