spark load data(online&offline), the blank value in the data will become null

**Bug Description**
When using load data to import data in CSV format, the empty string in the data will become null

select into result:

**Expected Behavior**

**Relation Case**
test_select_into_load_data.yaml id:0-1. 0-2、18-2、62

**Steps to Reproduce**
```sql
create table auto_YuMSwEvO(
id int,
c1 string,
c2 smallint,
c3 int,
c4 bigint,
c5 float,
c6 double,
c7 timestamp,
c8 date,
c9 bool,
index(key=(c1),ts=c7))options(partitionnum=1,replicanum=1);
insert into auto_YuMSwEvO values
(3,'',3,22,32,1.3,2.3,1590738991000,'2020-05-03',true);
create table auto_FwREKcjN(
id int,
c1 string,
c2 smallint,
c3 int,
c4 bigint,
c5 float,
c6 double,
c7 timestamp,
c8 date,
c9 bool,
index(key=(c1),ts=c7))options(partitionnum=1,replicanum=1);
set @@SESSION.execute_mode = "online";
select * from auto_YuMSwEvO into outfile '/Users/zhaowei/code/4paradigm/OpenMLDB/auto_YuMSwEvO.csv' ;
LOAD DATA INFILE 'file:///Users/zhaowei/code/4paradigm/OpenMLDB/auto_YuMSwEvO.csv' into table auto_FwREKcjN options(mode='append');
```