在不使用GSON或JSONObject的情况下，可以通过重写Converter.Factory来序列化null。以下是实现此方法的步骤：

1.创建NullOnEmptyConverterFactory.java类，并实现Converter.Factory接口。

import okhttp3.RequestBody;
import okhttp3.ResponseBody;
import retrofit2.Converter;
import retrofit2.Retrofit;

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;

public class NullOnEmptyConverterFactory extends Converter.Factory {
    @Override
    public Converter responseBodyConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
        final Converter delegate = retrofit.nextResponseBodyConverter(this, type, annotations);
        return new Converter() {
            @Override
            public Object convert(ResponseBody body) throws IOException {
                if (body.contentLength() == 0) return null;
                return delegate.convert(body);
            }
        };
    }

    @Override
    public Converter requestBodyConverter(Type type, Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
        final Converter delegate = retrofit.nextRequestBodyConverter(this, type, parameterAnnotations, methodAnnotations);
        return new Converter() {
            @Override
            public RequestBody convert(Object value) throws IOException {
                if (value == null) return RequestBody.create(okhttp3.MediaType.parse("text/plain"), "");
                return delegate.convert(value);
            }
        };
    }
}

2.在Retrofit实例中添加NullOnEmptyConverterFactory工厂。

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(baseUrl)
        .addConverterFactory(new NullOnEmptyConverterFactory())
        .build();

现在您可以在API响应中正常使用null。