@Override
    protected E map(final Traverser.Admin<S> traverser) {
        S traverserObject = traverser.get();
        return (E) ((traverserObject == null) ? null : String.valueOf(traverserObject));
    }
Reference implementation for asString() step, a mid-traversal step which returns the incoming traverser value
as a string. Null values are returned unchanged.

@author David Bechberger (http://bechberger.com)
@author Yang Xia (http://github.com/xiazcy)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/AsStringGlobalStep.java