WriteNullStringAsEmpty和WriteMapNullValue 两个序列化属性不能同时生效

[lenber]

问题：WriteNullStringAsEmpty和WriteMapNullValue 两个序列化属性不能同时生效

实例代码：

public static void main(String[] args) {
        Map<String, Object> jsonMap = new HashMap<>();
        jsonMap.put("a", 1);
        jsonMap.put("b", "");
        jsonMap.put("c", null);
        jsonMap.put("d", "哈哈");

        String str = JSONObject.toJSONString(jsonMap, SerializerFeature.WriteNullStringAsEmpty, SerializerFeature.WriteMapNullValue);
        System.out.println(str);
}

输出结果：
{"a":1,"b":"","c":null,"d":"哈哈"}

期望结果：
{"a":1,"b":"","c":"","d":"哈哈"}

版本：
1.2.83