fix: don't assume the member will always be included on MESSAGE_CREATE

This resolves #1153.

Member objects are only included on a message when the user has
transitioned from an offline state to an online state (i think?), so
this change will fall back to the prior behavior, where we just create
an incomplete member object for these states.

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1158,7 +1158,12 @@ namespace Discord.WebSocket
                                         if (author == null)
                                         {
                                             if (guild != null)
-                                                author = guild.AddOrUpdateUser(data.Member.Value); //per g250k, we can create an entire member now
+                                            {
+                                                if (data.Member.IsSpecified) // member isn't always specified...
+                                                    author = guild.AddOrUpdateUser(data.Member.Value); //per g250k, we can create an entire member now
+                                                else
+                                                    author = guild.AddOrUpdateUser(data.Author.Value); // user has no guild-specific data
+                                            }
                                             else if (channel is SocketGroupChannel)
                                                 author = (channel as SocketGroupChannel).GetOrAddUser(data.Author.Value);
                                             else