[usernotification] Add nullability to (generated and manual) bindings (#14941)

Co-authored-by: TJ Lambert <tjlambert@microsoft.com>

src/UserNotifications/UNNotificationAttachment.cs

@@ -7,6 +7,8 @@
 // Copyright 2016 Xamarin Inc. All rights reserved.
 //
 
+#nullable enable
+
 #if !TVOS
 using System;
 using Foundation;
@@ -14,7 +16,7 @@ using Foundation;
 namespace UserNotifications {
 	public partial class UNNotificationAttachment {
 
-		public static UNNotificationAttachment FromIdentifier (string identifier, NSUrl url, UNNotificationAttachmentOptions attachmentOptions, out NSError error)
+		public static UNNotificationAttachment? FromIdentifier (string identifier, NSUrl url, UNNotificationAttachmentOptions attachmentOptions, out NSError? error)
 		{
 			return FromIdentifier (identifier, url, attachmentOptions?.Dictionary, out error);
 		}