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.

indexShard != null is already checked above