Fix #1421 (in a better way) Return empty set when ActiveClients is null

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>