### Description

It'd be lovely is a `filters` or `filter` agg that touches a `constant_keyword` would properly rewrite itself to `match_all` and `match_none` and "go fast", at least when it's at the root of the aggs tree. Theoretically it should "just work" but it doesn't seem to. Also! The `terms` agg should be able to act on there being only a single possible value. Maybe it can even act on the `match_all`/`match_none` nature of the `constant_keyword`. I'm not sure how, but it does feel like there are optimizations in there. Particularly when `terms` is the top level agg.
server/src/test/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregatorTests.java
}
        withAggregator(builder, new MatchAllDocsQuery(), iw -> {}, (searcher, aggregator) -> {
            InternalFilters result = (InternalFilters) aggregator.buildEmptyAggregation();
            for (int i = 0; i < filters.length; i++) {
                assertThat(result.getBucketByKey(String.valueOf(i)).getDocCount(), equalTo(0L));
            }
            if (askForOtherBucket) {
                assertThat(result.getBucketByKey("other").getDocCount(), equalTo(0L));
            } else {
                assertThat(result.getBucketByKey("other"), nullValue());
            }
        });
    }
    public void testNoFilters() throws IOException {
        FiltersAggregationBuilder aggregationBuilder = new FiltersAggregationBuilder("test", new KeyedFilter[0]);
        testCase(
            iw -> { iw.addDocument(List.of()); },
            (InternalFilters result) -> { assertThat(result.getBuckets(), hasSize(0)); },
            new AggTestConfig(aggregationBuilder)
        );
    }
    public void testNoFiltersWithSubAggs() throws IOException {
        testCase(iw -> { iw.addDocument(List.of(new SortedNumericDocValuesField("i", 1))); }, (InternalFilters result) -> {
            assertThat(result.getBuckets(), hasSize(0));
        },
            new AggTestConfig(
                new FiltersAggregationBuilder("test", new KeyedFilter[0]).subAggregation(new MaxAggregationBuilder("m").field("i")),
                new NumberFieldMapper.NumberFieldType("m", NumberType.INTEGER)
            )
        );