Json transformation and problem with null values management in Java

I have a json string and I want to do some changes to it with the following code in Java using Jackson:

public String transformJson(String json) {

    final ObjectMapper mapper = new ObjectMapper();

    Student s = mapper.readValue(json, Student.class))

    // Do some changes on the object s

    return mapper.writeValueAsString(s);
}

Here is the Student class:

@AllArgsConstructor
@Data
@JsonInclude(Include.NON_NULL)
public static class Student {
    private String firstName;
    private String lastName;
    private String country;
    private Long age;
}

The code above works and let's suppose the json in input is the following:

{
   "firstName" : "Jim",
   "lastName" : "Miles"
}

If no change is done, the output would be the same. The annotation @JsonInclude(Include.NON_NULL) would just remove any null field from the output.

But now the new requirement is this: when the input json explicity define a value as null, then the output should show that field as null. That means that when the input is the following:

{
   "firstName" : "Jim",
   "lastName" : "Miles",
   "country" : null 
}

Then the output should still be:

{
   "firstName" : "Jim",
   "lastName" : "Miles",
   "country" : null 
}

Any idea on how I can achieve this?