- added support for empty aggregations. now the aggregator knows how to create an empty aggregation, for cases where empty buckets (and their sub-aggregations) need to be returned
- fixed a bug in histogram with "compute_empty_buckets" in cases where the execution was on a single shard
- fixed a bug in missing aggregator where it was bounded only to bytes base value context (now it works on value contexts of any type)
- fixed the aggregation context so values source will always be created as long as the value source config is valid. Now aggregators can be based on the generic ValueSource class of which values will be treated as bytes