fix: MessageUpdated without author (#1858)

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1359,6 +1359,8 @@ namespace Discord.WebSocket
                                     {
                                         //Edited message isnt in cache, create a detached one
                                         SocketUser author;
+                                        if (data.Author.IsSpecified)
+                                        {
                                             if (guild != null)
                                             {
                                                 if (data.WebhookId.IsSpecified)
@@ -1389,6 +1391,10 @@ namespace Discord.WebSocket
                                                     return;
                                                 }
                                             }
+                                        }
+                                        else
+                                            // Message author wasn't specified in the payload, so create a completely anonymous unknown user
+                                            author = new SocketUnknownUser(this, id: 0);
 
                                         after = SocketMessage.Create(this, State, author, channel, data);
                                     }