springboot返回前端对象null转为空字符串

在测试移动端接口的时候，手机端开发的人要求我们把返回对象null转成空字符串，觉得页面显示null不美观。虽然我让他们在手机端判断一下就好，不过他们很不情愿的样子，实际上就是懒。算了，我就上网查找了一下资料

import java.io.IOException;

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
/**
 * null返回空字符串
 * @author 戴旌旗
 *
 */
@Configuration
public class JacksonConfig {
    @Bean
    @Primary
    @ConditionalOnMissingBean(ObjectMapper.class)
    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        ObjectMapper objectMapper = builder.createXmlMapper(false).build();
        SerializerProvider serializerProvider = objectMapper.getSerializerProvider();
        serializerProvider.setNullValueSerializer(new JsonSerializer