TopHitsAggregationBuilder adds duplicate _score sort clauses

[soinio]

Elasticsearch version 6.5.4, also current master

Description of the problem:

When using High Level Rest Client Java API to produce search query, using
AggregationBuilders.topHits("th").sort("_score", SortOrder.DESC)
causes query to contain duplicate sort clauses:

                        "sort": [
                            {
                                "_score": {
                                    "order": "desc"
                                }
                            },
                            {
                                "_score": {
                                    "order": "desc"
                                }
                            }
                        ],

This also seems to break search, causing many scores to return as null.

It seems that this is because at the following locations ifs are missing their elses:
https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/search/aggregations/metrics/TopHitsAggregationBuilder.java#L233-L236
https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/search/aggregations/metrics/TopHitsAggregationBuilder.java#L250-L253

Simple work around is to use directly AggregationBuilders.topHits("th").sort(SortBuilders.scoreSort().order(SortOrder.DESC)).

[elasticmachine]

Pinging @elastic/es-analytics-geo

[Hohol]

It looked trivial, so I've made a PR #42179