Fix top_hits reduce error in case of empty shard results

The top_hits aggregation returned an empty InternalTopHits instance with no fields set when there were no result, causing reduce and serialization errors down the road. This is fixed by setting all required fields when a there are no results.

This bug does't affect any released versions, since the the top_hits aggregation is part of the upcoming and unreleased 1.3.0 release.