Stop aggregations from trying to build sub-aggregations that have never seen the a bucket #79381
Description
Right now aggregations like terms will blindly call buildAggregation for all buckets that might be collected whether or not there were any documents in that aggregation. That forces all sub-aggregations to add guards like
if (valuesSources == null || bucket >= counts.size()) {
But the parent aggregations already know if they've sent any documents to the sub-aggregations for a particular bucket. And they could just call buildEmptyAggregation for buckets that have never seen any docs. That'd be so much kinder.