@Override
    public void write(DocumentId id) {
        put(null, id.getScheme().toUtf8().getBytes());
        putByte(null, (byte) 0);
    }
Write out the value of weighted set field

@param field - field description (name and data type)
@param ws    - field value
document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer6.java