restore csv files failed with tidb backend

[glorv]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.
   Use lightning to restore some csv files.

Table schema:

CREATE TABLE `test` (
	`id` int(11) NOT NULL,
	`contract_no` varchar(64) DEFAULT NULL,
	`fund_seq_no` varchar(64) DEFAULT NULL,
	`term_no` int(11) DEFAULT NULL,
	`contract_type` varchar(8) DEFAULT NULL,
	`internal_transfer_tag` varchar(8) DEFAULT NULL,
	`prin_amt` int(11) DEFAULT NULL,
	`start_date` varchar(8) DEFAULT NULL,
	`end_date` varchar(8) DEFAULT NULL,
	`batch_date` varchar(8) DEFAULT NULL,
	`crt_time` timestamp DEFAULT CURRENT_TIMESTAMP,
	`region_code` varchar(8) DEFAULT NULL,
	`credit_code` varchar(64) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY RANGE COLUMNS(batch_date) (
	PARTITION `P20200224` VALUES LESS THAN ("2020-02-05 00:00:00"),
	PARTITION `P20200324` VALUES LESS THAN ("2020-03-05 00:00:00"),
	PARTITION `P20200424` VALUES LESS THAN ("2020-04-05 00:00:00"),
	PARTITION `P20200524` VALUES LESS THAN ("2020-05-05 00:00:00"),
	PARTITION `P_MAXVALUE` VALUES LESS THAN MAXVALUE
);

Some sample rows in the csv files:

contract_no,fund_seq_no,term_no,contract_type,internal_transfer_tag,prin_amt,start_date,end_date,region_code,credit_code
2020061030595,20200629,1,01,N,356,20200210,20200720,000000,
202023462,00628,1,01,N,3561,20200310,20200720,000001,
64030597,0627,1,01,N,3562,20200410,20200720,000002,33
64030598,626,12,02,Y,26368,20200510,20200620,000003,
030599,9520200625,1,01,N,3960,20200610,20200720,000005,999 

2. What did you expect to see?
   Lightning should import data successfully.

3. What did you see instead?
   Failed with following error:
   

4. Versions of the cluster
   v4.0.7

5. Operation logs

6. Configuration of the cluster and the task
   lightning config:
   

7. Screenshot/exported-PDF of Grafana dashboard or metrics' graph in Prometheus for TiDB-Lightning if possible