SpringBoot json null过滤为空字符串

@Configurationpublic class JacksonConfig {    @Bean    public Jackson2ObjectMapperBuilder objectMapperBuilder() {        Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();        builder.featuresToDisable(                SerializationFeature.WRITE_DATES_AS_TIMESTAMPS,                DeserializationFeature.ADJUST_DATES_TO_CONTEXT_TIME_ZONE);        return builder;    }    @Bean    @Primary    @ConditionalOnMissingBean(ObjectMapper.class)    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {        ObjectMapper objectMapper = builder.createXmlMapper(false).build();        objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer