* returns extra falsey values at the level of the wildcard

```
> obj
{ 
    foo: { 
        bar: { 
            qux: 'asdf' 
        },
        baz: null,
        ping: 'pong' 
    } 
}

> mask(obj,'foo/*/qux')
{ 
    foo: { 
        bar: { 
            qux: 'asdf' 
        },
        baz: null
    } 
}
```

Is this intended? I was expecting: 

```
{ 
    foo: { 
        bar: { 
            qux: 'asdf' 
        }
    } 
}
```
