@Override
    public E projectTraverser(final Traverser.Admin<S> traverser) {
        return null == traverser.get() ? null : (E) new MeanNumber(traverser.get(), traverser.bulk());
    }
Advances the starts until a non-null value is found or simply returns {@code null}. In this way, an all
{@code null} stream will result in {@code null}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MeanGlobalStep.java