fix extraction with other template variable

If there's other variable without I18n helper in template, error occurs.
Fix by falling back empty translation context into a plain object.

**Temaplte**:
```
<p>{{ I18n 'This is a test.' }}</p><p>{{ I18n 'This is another test' }}</p>
{{ test }}
```
**Error message**:
```
Warning: Cannot read property 'test' of undefined Use --force to continue.
```