解决mybatis plus字段为null或空字符串无法保存到数据库的问题

@TableField(strategy = FieldStrategy.IGNORED)

package com.wang.common.mybatis.handler;
import org.apache.ibatis.executor.result.ResultMapException;
import org.apache.ibatis.type.BaseTypeHandler;
import org.apache.ibatis.type.JdbcType;
import org.apache.ibatis.type.MappedJdbcTypes;
import org.apache.ibatis.type.MappedTypes;
import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
/**
 * @Name: CustomStringTypeHandler
 * @Desc: 自定义mybatis处理类，将null返回为空串（‘'）
 * @Author: Administrator
 * @Date: 2019-09-03 18:20
 */
@MappedTypes({String.class})
@MappedJdbcTypes(JdbcType.VARCHAR)
public class CustomStringTypeHandler extends BaseTypeHandler<String> {
  @Override
  public String getResult(ResultSet rs, String columnName) {
    String result;
    try {
      result = getNullableResult(rs, columnName);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set. Cause: " + e, e);
    }
    return result;
  }
  @Override
  public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)
      throws SQLException {
    ps.setString(i, parameter);
  }
  @Override
  public String getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    return rs.getString(columnName) == null? "" : rs.getString(columnName);
  }
  @Override
  public String getNullableResult(ResultSet rs, int columnIndex)
      throws SQLException {
    return rs.getString(columnIndex) == null? "" : rs.getString(columnIndex);
  }
  @Override
  public String getNullableResult(CallableStatement cs, int columnIndex)
      throws SQLException {
    return cs.getString(columnIndex) == null? "" : cs.getString(columnIndex);
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wang.sss.fw.mapper.BusinessTripMapper">
 <resultMap id="BaseResultMap" type="com.wang.sss.fw.pojo.BusinessTrip">
  <result column="REQUEST_ID" jdbcType="VARCHAR" property="requestId" />
  <result column="JOB_NUMBER" jdbcType="VARCHAR" property="jobNumber" />
  <result column="REQUEST_DATE" jdbcType="VARCHAR" property="requestDate" />
  <result column="DEPARTMENT" jdbcType="VARCHAR" property="department" />
  <result column="BUSINESS_DAYS" jdbcType="VARCHAR" property="businessDays"/>
  <result column="CFD" jdbcType="VARCHAR" property="cfd" />
  <result column="MDD" jdbcType="VARCHAR" property="mdd" />
  <result column="START_TIME" jdbcType="VARCHAR" property="startTime" />
  <result column="END_TIME" jdbcType="VARCHAR" property="endTime" />
  <result column="REASON" jdbcType="VARCHAR" property="reason" typeHandler="com.wang.common.mybatis.handler.CustomStringTypeHandler"/>
  <result column="REMARK" jdbcType="VARCHAR" property="remark" typeHandler="com.wang.common.mybatis.handler.CustomStringTypeHandler"/>
 </resultMap>
</mapper> 

BusinessTrip(requestId=11925, jobNumber=5721, requestDate=2019-05-06, department=57, businessDays=21, cfd=上海, mdd=南京, startTime=2019-05-06 13:36, endTime=2019-05-07 13:36, reason=null, remark=null)

BusinessTrip(requestId=11925, jobNumber=5721, requestDate=2019-05-06, department=57, businessDays=21, cfd=上海, mdd=南京, startTime=2019-05-06 13:36, endTime=2019-05-07 13:36, reason=, remark=)