Enable bindingDelegate getter without bind attr

Accessing the bindingDelegate of a <template> without a bind attribute causes horrible errors: 

```
TypeError: Cannot read property 'raw' of undefined
```

Culprit: https://github.com/Polymer/TemplateBinding/commit/23ef8f16ba0dc751d279cc179989851ec0ad00e1#diff-bb73baf1485cd281bcda48f60572ba1cR486

cc @rafaelw