@Override
    public StringFieldValue getStringFieldValue() {
        if (parent == null) {
            return null;
        }
        return parent.getStringFieldValue();
    }
Returns the StringFieldValue that this node belongs to, if any.

@return the StringFieldValue that this node belongs to, if any, otherwise null.
document/src/main/java/com/yahoo/document/annotation/SpanNode.java