@Override
    public E generateFinalResult(final E meanNumber) {
        // if the meanNumber is null it means the whole stream was null
        return null == meanNumber ? (E) null : (E) ((MeanNumber) meanNumber).getFinal();
    }
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