TIMESTAMP columns behavior under 11.4 and above

This is followup to https://jira.mariadb.org/browse/MCOL-5974

The test done there was for CREATE TABLE statement. It appears that ALTER TABLE statement can produce TIMESTAMP columns that have different NOT NULL checks for server (>-11.4) and for MCS.

One part of reproduction should create a table without TIMESTAMP column and add a column with TIMESTAMP type and use cpimport to import data (with timestamp) there.

The other part of reproduction should create a table and then alter a column to have TIMESTAMP value and then use cpimport to import data (with timestamp) into table.