code: public JSONObject elementOpt( String key, Object value, JsonConfig jsonConfig ) {
        verifyIsNull();
        if( key != null && value != null ){
           element( key, value, jsonConfig );
        }
        return this;
     }
docs: Put a key/value pair in the JSONObject, but only if the key and the value
  are both non-null.
  
  @param key A key string.
  @param value An object which is the value. It should be of one of these
  types: Boolean, Double, Integer, JSONArray, JSONObject, Long,
  String, or the JSONNull object.
  @return this.
  @throws JSONException If the value is a non-finite number.