Return a blank model, if what is passed in is undefined

@tobowers @yaymukund @xcoderzach 

```javascript
<div class='selectedQuestion-'>
  <span class='isName-'></span>
</div>
```

If there is no currently selected question, EndDash errors with 'cannot bind undefined to template'
Even wrapping above in `<div class='isSelectedQuestion-'>` errored

This PR fixes this with the logic, 'if the model scoped into is undefined, simply bind it to a blank Backbone model'. 

This:

a) Displays the correct boolean logic 
   It will not show anything inside of `<div class='isSelectedQuestion-'>`

b) Updates properly if `selectedQuestion` is set on the model in scope

c) Does not error

Negative is:

a) It might be harder to debug

Its worth noting, this is how EndDash behaves in release 0.3.3a<= 

There may be a better long term solution (don't bind reactions inside false conditionals) however this is a hard fix and requires more thinking. This is a good short-term solution that behaves correctly.