[Chapter 10] Claims manager assign page in .NET 7

[oleg-glushko]

I'm getting a validation error for an empty claim value when I recreate this page in a project targeting .NET 7. It says: "The Claim Value field is required." The BindProperty attribute's behavior has changed since .NET 6 and requires putting a question mark to make chapter 10 examples work.

[BindProperty, Display(Name = "Claim Value")]
public string? ClaimValue { get; set; }