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