Fixes the error thrown when using `{{input}}` as a block without label.

This is helpful for mobile pages, when you use placeholder text instead
of labels.

```html
{{#input username}}
  {{input-field username}}
  {{error-field username}}
{{/input}}
```
Merge pull request #92 from dockyard/alex-label

Fixes the error thrown when using `{{input}}` as a block without label.