fix: VoiceState#member nullable (#995)

* fix(types/voice): member field in VoiceState is nullable

* member is optional for live stages

* Update JSDoc comment block for VoiceState#member

* Revert "member is optional for live stages"

This reverts commit 2faa8b129b.

* feat(handlers/voice): add voiceLurkerJoin event

src/handlers/voice/VOICE_STATE_UPDATE.ts

@@ -9,6 +9,9 @@ export async function handleVoiceStateUpdate(data: DiscordGatewayPayload) {
   const payload = data.d as VoiceState;
   if (!payload.guildId) return;
 
+  // The "member" field in the payload is `null` if the user is in a live stage but not in the guild.
+  if (payload.member === null) return eventHandlers.voiceLurkerJoin?.(payload);
+
   const guild = await cacheHandlers.get("guilds", snowflakeToBigint(payload.guildId));
   if (!guild) return;