fix: #1421 (in a better way) Return empty set when ActiveClients is null (#1422)

This change ensures that SocketUser.ActiveClients will not return null, but instead an empty set by default. This can happen if the client has not recieved a presence update for a user, or if the user is not cached.

src/Discord.Net.WebSocket/Entities/Users/SocketUser.cs

@@ -40,7 +40,7 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         public UserStatus Status => Presence.Status;
         /// <inheritdoc />
-        public IImmutableSet<ClientType> ActiveClients => Presence.ActiveClients;
+        public IImmutableSet<ClientType> ActiveClients => Presence.ActiveClients ?? ImmutableHashSet<ClientType>.Empty;
         /// <summary>
         ///     Gets mutual guilds shared with this user.
         /// </summary>