fix: Fire GuildMemberUpdated without cached user (#1853)

src/Discord.Net.WebSocket/BaseSocketClient.Events.cs

@@ -347,6 +347,7 @@ namespace Discord.WebSocket
         }
         internal readonly AsyncEvent<Func<SocketUser, SocketUser, Task>> _userUpdatedEvent = new AsyncEvent<Func<SocketUser, SocketUser, Task>>();
         /// <summary> Fired when a guild member is updated, or a member presence is updated. </summary>
+        /// <remarks> The first user (state before) might be null if they weren't in the cache. </remarks>
         public event Func<SocketGuildUser, SocketGuildUser, Task> GuildMemberUpdated {
             add { _guildMemberUpdatedEvent.Add(value); }
             remove { _guildMemberUpdatedEvent.Remove(value); }