[tvservices] Add nullability to (generated and manual) bindings (#14928)

Co-authored-by: tj_devel709 <antlambe@microsoft.com>

src/TVServices/TVContentItemImageShape.cs

@@ -1,5 +1,7 @@
 // Copyright 2015 Xamarin Inc.
 
+#nullable enable
+
 #if !COREBUILD
 
 using System;