Object with 'or', keys contains an array with a 'null' element

``` javascript
joi.object().keys({
  a: joi.string(),
  b: joi.string()
}).or('a', 'b')
```

This is what I get if 'a' and 'b' doen't exist:

```
"validation": {
  "source":"payload",
  "keys": ["null"]
}
```

Should `keys` be an empty array? I'm using the master branch.
