fix(StageInstance): Ensure discoverableDisabled is not undefined

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
StageInstance#discoverableDisabled was possibly undefined.

This can occur via the following steps:

1. Start a stage channel instance
2. End the stage channel instance
3. Restart the bot to ensure that the instance is not cached
4. Fetch the audit log entry for this stage instance

Response

GuildAuditLogsEntry {
  targetType: 'STAGE_INSTANCE',
  actionType: 'DELETE',
  action: 'STAGE_INSTANCE_DELETE',
  reason: null,
  executor: User {},
  changes: [
    { key: 'topic', old: 'hi', new: undefined },
    { key: 'privacy_level', old: 2, new: undefined }
  ],
  id: '875086920567377920',
  extra: {
    channel: StageChannel {
      type: 'GUILD_STAGE_VOICE',
      deleted: false,
      guild: [Guild],
      guildId: '534940241002233896',
      parentId: '610956146450235414',
      permissionOverwrites: [PermissionOverwriteManager],
      id: '875085532114321409',
      name: 'hi',
      rawPosition: 0,
      rtcRegion: null,
      bitrate: 64000,
      userLimit: 10000,
      topic: null
    }
  },
  target: StageInstance {
    id: '875085574103511081',
    deleted: false,
    guildId: '534940241002233896',
    channelId: '875085532114321409',
    topic: 'hi',
    privacyLevel: 'GUILD_ONLY',
    discoverableDisabled: undefined
  }
}

This pull request asserts this property to null in the case of not knowing anything about this property.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating