Why JSON empty string deserializes to null at the server side?

I'm sending empty string through $.post and it deserializes to null. How to differentiate if the string was empty or null at the client side ?

Regards

UPDATE What I'm actually doing is:

$.post("Controller/Action", $.param({Name: ""}, true), null, "json");

at the server:

public Container
{
   public string Name;
}

public void Action(Container container)
{
    bool c = container.Name == null;   // c is true, why ?     
}