return an empty bops buffer if no chunks arrived

When using concat-stream I often assume that I'll get a `body` back that is a buffer, but if no chunks arrive I get this error:

```
            t.equal(body.toString('utf8'), row.used);
                         ^
TypeError: Cannot call method 'toString' of undefined
```

This patch just makes sure that I always get a buffer back if no data chunks arrived.