Sending null data in CustomPayload will cause all inventories to be filled with books and quills

Writing null itemstacks to a databuffer and sending it to the MC|BEdit or MC|BSign channel will result in the server replacing all player's inventories will books and quills, including future players that log on later.

Example of what all the players will see: https://i.imgur.com/JcuHRPi.png

Relies upon creative and applicable to certain servers (with protocollib?).
Adding a null check serverside could potentially fix this bug.