在达到高水位之前可用的缓冲区槽数量。如果写入器已关闭或消费者已断开连接，则返回 null。
null
🌐 The number of buffer slots available before the high water mark is reached. Returns null if the writer is closed or the consumer has disconnected.
该值始终为非负数。
🌐 The value is always non-negative.