Avoid declaring RazorPage<T>.Model as nullable by default (#39332) (#39416)

Contributes to https://github.com/dotnet/aspnetcore/issues/37510

src/Mvc/Mvc.Razor/src/PublicAPI.Unshipped.txt

 #nullable enable
+*REMOVED*Microsoft.AspNetCore.Mvc.Razor.RazorPage<TModel>.Model.get -> TModel?
+Microsoft.AspNetCore.Mvc.Razor.RazorPage<TModel>.Model.get -> TModel

src/Mvc/Mvc.Razor/src/RazorPageOfT.cs

@@ -15,13 +15,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         /// <summary>
         /// Gets the Model property of the <see cref="ViewData"/> property.
         /// </summary>
-        public TModel? Model => ViewData == null ? default(TModel) : ViewData.Model;
+        public TModel Model => ViewData.Model;
 
         /// <summary>
         /// Gets or sets the dictionary for view data.
         /// </summary>
         [RazorInject]
         public ViewDataDictionary<TModel> ViewData { get; set; } = default!;
-
     }
 }

src/Mvc/Mvc.ViewFeatures/src/PublicAPI.Unshipped.txt

 #nullable enable
+*REMOVED*Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary<TModel>.Model.get -> TModel?
+Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary<TModel>.Model.get -> TModel

src/Mvc/Mvc.ViewFeatures/src/ViewDataDictionaryOfT.cs

@@ -86,16 +86,10 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         }
 
         /// <inheritdoc />
-        public new TModel? Model
+        public new TModel Model
         {
-            get
-            {
-                return (base.Model == null) ? default(TModel) : (TModel)base.Model;
-            }
-            set
-            {
-                base.Model = value;
-            }
+            get => (base.Model is null) ? default! : (TModel)base.Model;
+            set => base.Model = value;
         }
     }
 }

src/ProjectTemplates/Web.ProjectTemplates/content/StarterWeb-CSharp/Views/Shared/Error.cshtml

@@ -6,10 +6,10 @@
 <h1 class="text-danger">Error.</h1>
 <h2 class="text-danger">An error occurred while processing your request.</h2>
 
-@if (Model?.ShowRequestId ?? false)
+@if (Model.ShowRequestId)
 {
     <p>
-        <strong>Request ID:</strong> <code>@Model?.RequestId</code>
+        <strong>Request ID:</strong> <code>@Model.RequestId</code>
     </p>
 }