@Override
        public Double next() {
            try {
                return get(count++);
            }
            catch (IllegalArgumentException e) {
                throw new NoSuchElementException("No element at position " + count);
            }
        }
Fill the given list with nulls if necessary to make sure it has a (possibly null) value at the given index
vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java