Fix errors caused by missing signature in packet player_chat

[u9g]

Replaces #1509 (original by @TAOtxi, branch became unpushable after maintainer edits were disabled).

Problem

On offline servers (tested on 1.21.11), running bot.chat('/w xx something') causes the bot to receive a player_chat packet without a signature attribute, which raises an error at src/client/chat.js:199:

const verified = !packet.unsignedChatContent && updateAndValidateSession(packet.senderUuid, packet.plainMessage, packet.signature, packet.index, packet.previousMessages, packet.salt, packet.timestamp) && !expired

packet.signature is declared as option in the protocol data, so it is undefined when the server does not sign the message. updateAndValidateSession then passes it to crypto.verify('RSA-SHA256', ...), which throws a TypeError and kills the packet handler before playerChat is emitted.

Fix

Short-circuit on a missing signature before attempting validation, matching vanilla behavior:

• PlayerChatMessage#verify null-checks the signature before verifying (this.signature != null && ...)
• ChatTrustLevel#evaluate treats a missing signature as NOT_SECURE — the message is still displayed, just flagged; it is never an error condition

With this change, verified is false for unsigned messages and the playerChat event still fires.

The same guard is not needed in the 1.19.1 chainedChatWithHashing branch, where signature is declared as a required buffer in the protocol data.