Fixed nullable annotation. (#29389)

Co-authored-by: James Newton-King <[email protected]>

Author: ShreyasJejurkar

src/Security/Authentication/Cookies/src/ITicketStore.cs

@@ -30,8 +30,8 @@ public interface ITicketStore
         /// Retrieves an identity from the store for the given key.
         /// </summary>
         /// <param name="key">The key associated with the identity.</param>
-        /// <returns>The identity associated with the given key, or if not found.</returns>
-        Task<AuthenticationTicket> RetrieveAsync(string key);
+        /// <returns>The identity associated with the given key, or <c>null</c> if not found.</returns>
+        Task<AuthenticationTicket?> RetrieveAsync(string key);
 
         /// <summary>
         /// Remove the identity associated with the given key.

src/Security/Authentication/Cookies/src/PublicAPI.Shipped.txt

@@ -80,7 +80,7 @@ Microsoft.AspNetCore.Authentication.Cookies.ICookieManager.GetRequestCookie(Micr
 Microsoft.AspNetCore.Authentication.Cookies.ITicketStore
 Microsoft.AspNetCore.Authentication.Cookies.ITicketStore.RemoveAsync(string! key) -> System.Threading.Tasks.Task!
 Microsoft.AspNetCore.Authentication.Cookies.ITicketStore.RenewAsync(string! key, Microsoft.AspNetCore.Authentication.AuthenticationTicket! ticket) -> System.Threading.Tasks.Task!
-Microsoft.AspNetCore.Authentication.Cookies.ITicketStore.RetrieveAsync(string! key) -> System.Threading.Tasks.Task<Microsoft.AspNetCore.Authentication.AuthenticationTicket!>!
+Microsoft.AspNetCore.Authentication.Cookies.ITicketStore.RetrieveAsync(string! key) -> System.Threading.Tasks.Task<Microsoft.AspNetCore.Authentication.AuthenticationTicket?>!
 Microsoft.AspNetCore.Authentication.Cookies.ITicketStore.StoreAsync(Microsoft.AspNetCore.Authentication.AuthenticationTicket! ticket) -> System.Threading.Tasks.Task<string!>!
 Microsoft.AspNetCore.Authentication.Cookies.PostConfigureCookieAuthenticationOptions
 Microsoft.AspNetCore.Authentication.Cookies.PostConfigureCookieAuthenticationOptions.PostConfigure(string! name, Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationOptions! options) -> void