Removed content response from NotFound condition

If any content is returned as part of a result—as opposed to just an HTTP header—then the `UseStatusCodePages()` won't intercept the request. This fixes that issue.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/_filters/ValidateTopicAttribute.cs

@@ -75,7 +75,7 @@ public override void OnActionExecuting(ActionExecutingContext context) {
 
       if (currentTopic is null) {
         if (!AllowNull) {
-          context.Result = controller.NotFound("There is no topic associated with this path.");
+          context.Result = controller.NotFound();
         }
         return;
       }