Remove `DEFAULT NULL` for primary key column to support MySQL 5.7.3

Since MySQL 5.7.3 m13 does now allow primary key column is null.
Merge pull request #13247 from yahonda/support_mysql573_primary_key

From http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-3.html:
"Columns in a PRIMARY KEY must be NOT NULL, but if declared explicitly as NULL produced no error. Now an error occurs."