Federated aggregates get tripped up by zero hits from one source

```
If you try a query like:

select ?s (count(?o) as ?c) { ?s <prop> ?o } group by ?s

Over two sources where, in one, prop doesn't appear you get a result like:

< a, 2 > < b, 1 > < NULL, 0 >

due to one having no hits and hence grouping incorrectly.




```

Original issue reported on code.google.com by `pl...@mac.com` on 18 Mar 2011 at 1:06
