Handle null PreferredLocale in rare cases where Discord messes up

[hollowstrawberry]

Sometimes Discord messes up and leaves a guild with a null PreferredLocale, causing an error to be thrown. This fixes that from Discord.Net's end even though it's Discord's fault.

I made this this PR because I had been getting an error for a single guild every time my bot starts up.

Gateway6> Error handling Dispatch (GUILD_AVAILABLE): System.ArgumentNullException: Value cannot be null. (Parameter 'name')
   at System.Globalization.CultureInfo..ctor(String name, Boolean useUserOverride)
   at Discord.WebSocket.SocketGuild.Update(ClientState state, Guild model)
   at Discord.WebSocket.SocketGuild.Update(ClientState state, ExtendedGuild model)
   at Discord.WebSocket.DiscordSocketClient.ProcessMessageAsync(GatewayOpCode opCode, Nullable`1 seq, String type, Object payload)

[hollowstrawberry]

Actually I found the guild, I guess I'll make a report to the discord api

ID is 297408095137562625