how to perform checksum if number of levels is less than 25

[caoyang1024]

Issue type

• bug

Brief description

according to docs,

The Websocket V2 Books and Raw Books support the option to request that checksums be sent through the WebSocket connection. The checksum is a CRC32 value which covers the first 25 bids and asks. By calculating your own checksum and comparing it to the provided value, you can verify that your order book data is correct and up to date.

but when count is 0, and level is deleted, how can i still perform the checksum calculation?