Internal: fixed wrong version check s/onOrBefore/before in TransportShardSingleOperationAction

relates to #7223
Internal: Add some @Nullable annotations and fix related compilation warnings.

Added @Nullable to:
 - IndicesService.indexService
 - IndexService.shard
 - IndexService.shardInjector

This change doesn't try to do anything smart but just makes sure that a
*MissingException is thrown instead of a NullPointerException when the requested
object doesn't exist.

Close #7251
[TESTS] Make DisabledFieldDataFormatTests less evil.
function_score: fix explanation, [ was missing
[TEST] remove unneeded explain(true)

closes #7248
Fixed a typo