Cannot create ChangeNotificationCollection from JSON

[jasonjoh]

Creating a ChangeNotificationCollection from a JSON stream using CreateFromDiscriminatorValue results in an object with all fields set to null. This seems to be caused by the BackingStore never getting initialized, since there is no constructor for this class.

Note this issue also exists in the Beta SDK, see microsoftgraph/msgraph-beta-sdk-dotnet#730

Repro code

using Microsoft.Graph.Models;
using Microsoft.Kiota.Serialization.Json;

var json = @"{
    ""value"": [
        {
            ""changeType"": ""updated"",
            ""subscriptionId"": ""c224ac82-d3f3-4079-80ed-b6cd355e0f56"",
            ""resource"": ""external"",
            ""clientState"": null,
            ""resourceData"": {
                ""@odata.type"": ""#Microsoft.Graph.connector"",
                ""@odata.id"": ""external"",
                ""id"": ""cf6c33ae-e462-41c8-928b-88a8dd410f23"",
                ""state"": ""enabled"",
                ""connectorsTicket"": ""eyJhbGciOiJIU...""
            },
            ""subscriptionExpirationDateTime"": ""2023-09-20T20:39:46.8577199+00:00"",
            ""tenantId"": ""cf6c33ae-e462-41c8-928b-88a8dd410f23""
        }
    ],
    ""validationTokens"": [
        ""eyJ0eXAiOiJKV1...""
    ]
}";

using var memStream = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(json));

var parseNode = new JsonParseNodeFactory().GetRootParseNode("application/json", memStream);
var changeNotifications = parseNode.GetObjectValue(ChangeNotificationCollection.CreateFromDiscriminatorValue);

Console.WriteLine($"changeNotifications.Value: {(changeNotifications?.Value == null ? "NULL" : changeNotifications.Value.Count)}");
Console.WriteLine($"changeNotifications.ValidationTokens: {(changeNotifications?.ValidationTokens == null ? "NULL" : changeNotifications.ValidationTokens.Count)}");