message: Let's have an early out in case source is empty
diff: @@ -70,6 +70,9 @@ template <class T> struct QuantileOperation {
  }
  template <class STATE, class OP> static void Combine(STATE source, STATE *target) {
  + if (source.pos == 0) {
  + return;
  + }
  resize_state(target, target->pos + source.pos);
  memcpy(target->v + target->pos * sizeof(T), source.v, source.pos * sizeof(T));
  target->pos += source.pos;
  