Ref and Alternatives.when don't handle empty string keys

Both of these schemas will throw assertion errors because `ref` is falsy:

```
Joi.object({
    a: Joi.alternatives().when('', { is: 5, then: Joi.valid('y') }),
    '': Joi.any()
});
```

and 

```
Joi.object({
    a: Joi.ref(''),
    '': Joi.any()
});
```
