Map a JSON field that can have different types with Jackson?

I get JSON from a web service and can not influence the JSON format. The JSON code below is just an example to illustrate the problem. The field cars can either be an object containing Car objects or it can be an empty string. If I could change the web service, I'd change the empty String to be an empty object like "cars" : {} instead of "cars" : "".

When trying to map JSON to this Java object:

public class Person {
    public int id;
    public String name;
    public Map<String, Car> cars;
}

This works:

{
    "id" : "1234",
    "name" : "John Doe",
    "cars" : {
        "Tesla Model S" : {
            "color" : "silver",
            "buying_date" : "2012-06-01"
        },
        "Toyota Yaris" : {
            "color" : "blue",
            "buying_date" : "2005-01-01"
        }
    }
}

And this fails:

{
    "id" : "1",
    "name" : "The Dude",
    "cars" : ""
}

What would be the best way to handle this case in Jackson? If there's the empty string, I'd like to get null for the field cars. I tried using ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, but it didn't help.