made IndicesRelatedRequestHelper a static inner class within IndicesRelatedRequest
renamed relatedIndices method to requestedIndices
switched to IllegalStateException in case a put warmer doesn't contain a search request
use an assert instead of exception if a write request within a bulk relates to more than one index
added asserts to state that the single index should never be null for some internal requests
make sure NPE is never thrown when setting indices(null) to IndicesAliasesRequest, similar to what SearchRequest does

Also make sure that requestedIndices doesn't accept empty indices, which are not accepted during validation either.
make sure null index is not accepted by MoreLikeThisRequest#requestedIndices, illegal state is thrown