Joi.any().when is not evaluated if empty

Joi.any().when is not evaluated it its value is empty even if the when condition is fulfilled.

For example: 

``` javascript
 Joi.object({
   a: Joi.string().when('b', {is: true, then: Joi.required()}),
   b: Joi.boolean().default(false)
})
```

If you use this object to validate {b: true} the returned value will remain {b: true} but it actually should return an error. 
