CS8620: Differences in the nullability of reference types

With the tests migrated to .NET 6, we have improved nullability annotations on the core libraries, and thus the tests now recognize that `object` type parameter for the `actionArguments` is nullable. As a result, the tests are updated to align these types.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Tests/ValidateTopicAttributeTest.cs

@@ -44,7 +44,7 @@ public static ActionExecutingContext GetActionExecutingContext(Controller contro
       var actionExecutingContext = new ActionExecutingContext(
         actionContext,
         new List<IFilterMetadata>(),
-        new Dictionary<string, object>(),
+        new Dictionary<string, object?>(),
         controller
       );