Fix error 3140 'Invalid JSON text' when inserting MAP literal '{1:NULL}'

Description

When inserting a MAP literal '{1:NULL}' (with NULL in uppercase), the operation fails with error 3140: "Invalid JSON text".

Actual Behavior:
The INSERT statement fails.

Expected Behavior:
The INSERT statement should succeed, similar to the lowercase null literal ('{1:null}') which works correctly.

Steps to Reproduce

1. Access the machine.
2. Navigate to the directory: /obdata/data/AndroidTest/ob_opensource_test/tools/deploy
3. Set up the test environment and run the specific mysqltest case using the following commands:

export OCEANBASE_HOME=/obdata/data/AndroidTest/ob_opensource_test/
cd ${OCEANBASE_HOME}/tools/deploy
source activate_obd.sh
export OBD_SEEKDB_STATUS_FALLBACK_TCP=1

obd test mysqltest test_env --mysqltest-bin=${OCEANBASE_HOME}/deps/3rd/u01/obclient/bin/mysqltest --obclient-bin=${OCEANBASE_HOME}/deps/3rd/u01/obclient/bin/obclient --init-only --init-sql-dir=${OCEANBASE_HOME}/tools/deploy --init-sql-files='init.sql,init_user.sql|root@sys|test'
obd test mysqltest test_env --mysqltest-bin=${OCEANBASE_HOME}/deps/3rd/u01/obclient/bin/mysqltest --obclient-bin=${OCEANBASE_HOME}/deps/3rd/u01/obclient/bin/obclient --disable-reboot --test-set=map.map_keys_mysql

4. The issue can be reproduced with the following SQL statements:

USE test;

DROP TABLE IF EXISTS t4;
CREATE TABLE t4 (
  id INT NOT NULL AUTO_INCREMENT,
  m MAP(INT, INT[])
);
INSERT INTO t4(m) VALUES ('{1:[1],2:[2],3:[3]}');
INSERT INTO t4(m) VALUES ('{1:NULL}'); -- This line fails with error 3140.
-- Lowercase 'null' works correctly.
INSERT INTO t4(m) VALUES ('{1:null}');

Start in /obdata/data/AndroidTest/ob_opensource_test/tools/deploy and run the mysqltest case map.map_keys_mysql with the provided setup commands. Compare the INSERT statements using uppercase NULL and lowercase null, then trace the MAP literal handling reached by that test. Done means '{1:NULL}' inserts successfully without error 3140 while the existing lowercase case continues to work.