The shardTopDocs array should get created with the size equal to the total number of shard level requests and not the total number of requests that have a shard level result.
Make sure no null TopDocs entires are passed down to TopDocs#merge
Added dedicated scroll tests that tests scrolling on an index that has missing shards due to node failure.
Made sure that the sort fields in SimpleNestedTests exists by adding the fields in the mapping during index creation.