Fix sort() bug when ids.length is 0.

When ids.length is 0, sort() results the whole records in the database.

Here's the source code.
```
  models.newsfeed_status.find(
      {
        // some search parameters
      },
      function(err, ids) {  
        models.newsfeed_status.sort(
        {
            field: 'written_date',
            direction: 'DESC'
        }, ids /* HERE */, function(err, statuses){ 
            // some code
        }); 
      }
  );
```

This pull request fixes #74.
Please review this.
Thank you for reading.