@Override
    protected S generateSeedFromStarts() {
        S s = null;
        while (starts.hasNext() && null == s) {
            s = projectTraverser(this.starts.next());
        }

        return s;
    }
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/SumGlobalStep.java