* Check whether specified List is null and if so return an immutable list instead. This method supports
* non-null default getter methods.
*
+ * @param <T> list element type
* @param input input list, may be null
* @return Input list or an empty list.
*/
* Check whether specified Map is null and if so return an immutable map instead. This method supports
* non-null default getter methods.
*
+ * @param <K> key type
+ * @param <V> value type
* @param input input map, may be null
* @return Input map or an empty map.
*/