Internal: fixed bw comp in ShardSearchTransportRequest

Original indices were introduced in 1.4.0.Beta1 as optional, preceded by a boolean flag that told whether they were empty or not. We have to keep serializing as optional only when reading and writing from/to 1.4.0.Beta1, although the original indices are not optional whenever we go and serialize the request they belong to (which is why the boolean got removed in 1.4.0).