[bug] Invalid default value for timestamp

[hustjieke]

bug:

mysql> create table tx(a timestamp null default null, b int key);
ERROR 1067 (42000): Invalid default value for 'a'

After radon parse sql
create table tx(a timestamp null default null, b int key);
and send to backends, the sql will be changed to
create table tx(a timestamp default null, b int key);.
For type timestamp TIMESTAMP fields are NOT NULL by default in 5.7, so we should not remove null after parse.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.