Empty post-aggs serialization

[QubitPi]

If a query does not have post aggregation spec, it still includes it as an empty spec. This wastes network resources and reduces the readability of Druid query.

{
    "queryType" : "...",
    "dataSource" : ...,
    "granularity" : ...
    "intervals" : ...,
    "dimensions" : [ ... ],
    "aggregations" : [ ... ],
    "postAggregations" : [ ]
  }

[michael-mclawhorn]

I was recently reminded that making this change effective immediately will break tests. We need to feature flag this or in some other way make this not effective immediately and provide migration notes.

This actually highlights that making some partial-query matching tests (as opposed to our full query matching tests) would be a big help for both fili testing and downstream testing.