@Override
    public void processAllStarts() {
        if (this.starts.hasNext())
            super.processAllStarts();
    }
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