invalid comparison: java.lang.String and java.util.Date

在xml中如果将时间类型和空字符串""比较，会引发此错误，只保留null判断即可

<if test="null != startDate and startDate != '' ">
    start_date=#{startDate,jdbcType=DATE},
</if>

<if test="null != startDate">
    start_date=#{startDate,jdbcType=DATE},
</if>