ETag property throws null ref exception when there's no raw response available (4.0.0-preview3)

Describe the bug Inheriting from ItemResponse<T> class and not overriding ETag property in the derived class will cause null reference exception being thrown when you try to access the ETag field despite it being nullable.

Looking at the source code, this seems to happen when there is no raw response available; https://github.com/Azure/azure-cosmos-dotnet-v3/blob/releases/4.0.0-preview3/Microsoft.Azure.Cosmos/azuredata/Resource/Container/ItemResponse.cs#L70

To Reproduce

public class MockItemResponse<T> : ItemResponse<T>
{
    public MockItemResponse(T value) { }
}

class Program
{
    static void Main(string[] args)
    {
        var response = new MockItemResponse<string>("test");

        var test1 = response.ETag.HasValue; // Null ref exception
        var test2 = response.ETag == null; // Null ref exception
        var test3 = response?.ETag == null; // Null ref exception
    }
}

Expected behavior Return null.

Actual behavior NullReferenceException is being thrown.

Environment summary SDK Version: 4.0.0-preview3 OS Version: Windows

Additional context I assume this it not an issue you run into in normal use, but it is a bit unexpected behavior when writing mocks for unit testing purposes, given that the property is marked as nullable.

[j82w]

@Jaah thanks for reporting this. It should be updated to do the following.

this.GetRawResponse()?.Headers?.ETag;

[ealsur]

Closing as mocking is possible and ETag is always expected on the real scenario