Fix NRE in modify guild channel

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -387,8 +387,12 @@ namespace Discord.API
             Preconditions.NotNull(args, nameof(args));
             Preconditions.AtLeast(args.Position, 0, nameof(args.Position));
             Preconditions.NotNullOrWhitespace(args.Name, nameof(args.Name));
-            Preconditions.LessThan(args.Name.Value.Length, 100, nameof(args.Name));
-            Preconditions.LessThan(args.Topic.Value.Length, 1024, nameof(args.Name));
+
+            if(args.Name.IsSpecified)
+                Preconditions.LessThan(args.Name.Value.Length, 100, nameof(args.Name));
+            if(args.Topic.IsSpecified)
+                Preconditions.LessThan(args.Topic.Value.Length, 1024, nameof(args.Name));
+
             Preconditions.AtLeast(args.SlowModeInterval, 0, nameof(args.SlowModeInterval));
             Preconditions.AtMost(args.SlowModeInterval, 21600, nameof(args.SlowModeInterval));
             options = RequestOptions.CreateOrClone(options);