Marked VideoUrl as non-nullable, required

This is similar to the updates made previously to core properties (1bf25be). `VideoUrl` is a core property of the `VideoTopicViewModel`, and always expected to have a value. If it doesn't, a validation error should be thrown.

Author: JeremyCaney

OnTopic.ViewModels/_contentTypes/VideoTopicViewModel.cs

@@ -4,6 +4,8 @@
 | Project       Topics Library
 \=============================================================================================================================*/
 using System;
+using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
 
 namespace OnTopic.ViewModels {
 
@@ -26,6 +28,7 @@ public record VideoTopicViewModel: PageTopicViewModel {
     /// <summary>
     ///   Provides a URL reference to a video to display on the page.
     /// </summary>
+    [Required, NotNull, DisallowNull]
     public Uri? VideoUrl { get; init; }
 
     /*==========================================================================================================================