default value for timestamp

[KenjiOhtsuka]

Still, I'm wondering how to handle Timestamp NOT NULL constraint,
because MySQL 5.5.6 and later handles Timestamp NOT NULL as default.
Current program handle nullable = true for TimestampColumn as default TIMESTAMP column in MySQL,
so it become NOT NULL. On the other hand, PostgreSQL handles plain Timestamp column nullable.

I think it is better to force nullable in MySQL when TimestampColumn has nullable = true.
The issue is the default nullable value in program, timestamp function in TableBuilder and addTimestampColumn in AbstractMigration