Throw from IUserIdKeyResolver if id/key not found. (#14101)

* Throw from IUserIdKeyResolver if id/key not found. This is too critical to not throw.

* Fixed tests

* Explicitly test that we can resolve super user key/ID from their counterparts

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Co-authored-by: kjac <kja@umbraco.dk>

src/Umbraco.Core/Services/IUserIdKeyResolver.cs

@@ -7,12 +7,12 @@ public interface IUserIdKeyResolver
     /// </summary>
     /// <param name="key">The key of the user. </param>
     /// <returns>The id of the user, null if the user doesn't exist.</returns>
-    public Task<int?> GetAsync(Guid key);
+    public Task<int> GetAsync(Guid key);
 
     /// <summary>
     /// Tries to resolve a user id to a user key without fetching the entire user.
     /// </summary>
     /// <param name="id">The id of the user. </param>
     /// <returns>The key of the user, null if the user doesn't exist.</returns>
-    public Task<Guid?> GetAsync(int id);
+    public Task<Guid> GetAsync(int id);
 }