Fixed null author when downloading messages for an uncached user

src/Discord.Net/Entities/WebSocket/MessageCache.cs

@@ -116,7 +116,11 @@ namespace Discord
                     RelativeMessageId = relativeId
                 };
                 var downloadedMessages = await _discord.ApiClient.GetChannelMessagesAsync(_channel.Id, args).ConfigureAwait(false);
-                return cachedMessages.Concat(downloadedMessages.Select(x => new CachedMessage(_channel, _channel.GetUser(x.Id), x))).ToImmutableArray();
+                return cachedMessages.Concat(downloadedMessages.Select(x =>
+                {
+                    var user = _channel.GetUser(x.Id) ?? new User(_channel.Discord, x.Author.Value) as IUser;
+                    return new CachedMessage(_channel, user, x);
+                })).ToImmutableArray();
             }
         }
     }