Groonga's select doesn't return an empty array for empty result. It just returns nothing.

```
groonga -n /tmp/db
> table_create X
[[0,1384827709.96056,0.0471649169921875],true]
> select X
[[0,1384827711.30404,0.000353574752807617],[[[0],[["_id","UInt32"]]]]]
> 
```