code: public static <T> List<T> emptyListIfNull(final List<T> list) {
  		return list == null ? (List<T>) Collections.EMPTY_LIST : list;
  	}
docs: 如果list为null，转化为一个安全的空List.
  
  注意返回的List不可写, 写入会抛出UnsupportedOperationException.
  
  @see java.util.Collections#emptyList()