A number of changes to fix reduce failures if shard failures have occurred:
* 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.

Closes #6022