Not able to update Null values in elastic search using Nest with field mapping

[prvn_prasher]

I just noticed I couldn't able to update null values in existing properties. For example, i am trying to update the Date.
I am using Nest with C# with field mapping.
So I think serialization ignoring null values.
I tried the serialization and shown it in the docs.
but I am getting exceptions
```
[Text(Name = "fd_status", Fielddata = true)]
public string FirstDraftStatusValue { get; set; }

    /// <summary>
    /// Gets or sets the FirstDraftClearedDate.
    /// </summary>
    [Date(Name = "fd_3p_date")]
    public DateTime? FirstDraftTPUpdateDate { get; set; }

    /// <summary>
    /// Gets or sets the first cleared draft date.
    /// </summary>
    [Date(Name = "fcd_date")]

    public DateTime? FirstClearedDraftDate { get; set; }

     ```
var uri = new Uri(this.hostUrl);
              
                        var pool = new SingleNodeConnectionPool(uri);
                        var connectionSettings =
                            new ConnectionSettings(pool, sourceSerializer: (builtin, settings) => new JsonNetSerializer(
                                builtin, settings,
                                () => new JsonSerializerSettings { NullValueHandling = NullValueHandling.Include },
                                resolver => resolver.NamingStrategy = new SnakeCaseNamingStrategy()
                            ));

                        elasticClient = new ElasticClient(connectionSettings);

Exception : Could not load type 'Nest.CartesianPoint' from assembly 'Nest, Version=7.0.0.0, Culture=neutral, PublicKeyToken=96c599bbe3e70f5d'.