fix: NRE with Cacheable.DownloadAsync() (#2331)

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1305,13 +1305,13 @@ namespace Discord.WebSocket
 
                                             user.Update(State, data);
 
-                                            var cacheableBefore = new Cacheable<SocketGuildUser, ulong>(before, user.Id, true, () => null);
+                                            var cacheableBefore = new Cacheable<SocketGuildUser, ulong>(before, user.Id, true, () => Task.FromResult<SocketGuildUser>(null));
                                             await TimedInvokeAsync(_guildMemberUpdatedEvent, nameof(GuildMemberUpdated), cacheableBefore, user).ConfigureAwait(false);
                                         }
                                         else
                                         {
                                             user = guild.AddOrUpdateUser(data);
-                                            var cacheableBefore = new Cacheable<SocketGuildUser, ulong>(null, user.Id, false, () => null);
+                                            var cacheableBefore = new Cacheable<SocketGuildUser, ulong>(null, user.Id, false, () => Task.FromResult<SocketGuildUser>(null));
                                             await TimedInvokeAsync(_guildMemberUpdatedEvent, nameof(GuildMemberUpdated), cacheableBefore, user).ConfigureAwait(false);
                                         }
                                     }