Set user attribute to null or string.empty

I'm trying to set the value of an Azure AD user to null or string.Empty, by using the Microsoft.Graph NuGet Package in a C# app.

That's my code:

public async Task<User> UpdateMobilePhone(string userId, string mobilePhone)
    {
        User user = new User();
        user.MobilePhone = mobilePhone;

        return await graphServiceClient.Users[userId]
            .Request()
            .UpdateAsync(user);
    }

If a set the value of "mobilePhone" to string.Empty I get the error: Microsoft.Graph.ServiceException: "Code: Request_BadRequest Message: Invalid value specified for property 'mobilePhone' of resource 'User'.

And if I set the value of "mobilePhone" to null it's happening nothing because the default value of a empty user is null so he thinks, that nothing is changed.

So does someone know how I can resolve my problem?

Thank you in forward