@Amit_Saxena Try following commands
bench --site <your-site-name> mariadb
In the mariadb console fire following query
update `tabLeave Type` set max_days_allowed = '0' where trim(coalesce(max_days_allowed, '')) = '';
.
.
.
Then continue with your migration
bench --site <your-site-name> migrate
Best,
Suraj