@Override
    public void writeVertices(final OutputStream outputStream, final Iterator<Vertex> vertexIterator) throws IOException {
        writeVertices(outputStream, vertexIterator, null);
    }
Writes a list of vertices without edges.

@param outputStream the stream to write to.
@param vertexIterator    a iterator that returns a list of vertices.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphson/GraphSONWriter.java