Don't throw when receiving a presence for a user that doesn't exist

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -157,8 +157,6 @@ namespace Discord.WebSocket
                 {
                     if (members.TryGetValue(model.Presences[i].User.Id, out SocketGuildUser member))
                         member.Update(state, model.Presences[i], true);
-                    else
-                        Debug.Assert(false);
                 }
             }
             _members = members;
@@ -242,8 +240,6 @@ namespace Discord.WebSocket
                 {
                     if (members.TryGetValue(model.Presences[i].User.Id, out SocketGuildUser member))
                         member.Update(state, model.Presences[i], true);
-                    else
-                        Debug.Assert(false);
                 }
             }
             _members = members;