Fix nullref when passing null to GetShardIdFor. (#1049)

src/Discord.Net.WebSocket/DiscordShardedClient.cs

@@ -129,7 +129,7 @@ namespace Discord.WebSocket
         private int GetShardIdFor(ulong guildId)
             => (int)((guildId >> 22) % (uint)_totalShards);
         public int GetShardIdFor(IGuild guild)
-            => GetShardIdFor(guild.Id);
+            => GetShardIdFor(guild?.Id ?? 0);
         private DiscordSocketClient GetShardFor(ulong guildId)
             => GetShard(GetShardIdFor(guildId));
         public DiscordSocketClient GetShardFor(IGuild guild)