Fail fast if authenticator key is null (#14037)

Author: HaoK

src/Identity/Extensions.Core/src/AuthenticatorTokenProvider.cs

@@ -48,7 +48,7 @@ public virtual async Task<bool> ValidateAsync(string purpose, string token, User
         {
             var key = await manager.GetAuthenticatorKeyAsync(user);
             int code;
-            if (!int.TryParse(token, out code))
+            if (key == null || !int.TryParse(token, out code))
             {
                 return false;
             }