Fix the schema migration bug

[noahingh]

A schema migration error occurs for the MySQL database. The error is that while upgrading the Ent, the default value of the foreign key constraint has changed(#381). To avoid this, rever to the same value as before.

migrate the table schema: sql/schema: alter table "notification_records": Error 1830: Column 'event_id' cannot be NOT NULL: needed in a foreign key constraint 'notification_records_events_notification_record' SET NULL