The table structure from MySQL-5.1.14 is:

CREATE TABLE `slow_log` (
  `start_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `user_host` mediumtext NOT NULL,
  `query_time` time NOT NULL,
  `lock_time` time NOT NULL,
  `rows_sent` int(11) NOT NULL,
  `rows_examined` int(11) NOT NULL,
  `db` varchar(512) DEFAULT NULL,
  `last_insert_id` int(11) DEFAULT NULL,
  `insert_id` int(11) DEFAULT NULL,
  `server_id` int(11) DEFAULT NULL,
  `sql_text` mediumtext NOT NULL
) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='Slow log'

Even as far back as MySQL-5.5.40 this table could be created as NULLs
where not permitted in the CSV table time, but it seems they
where allowed sometime.

As the first part of mariadb-upgrade adds the column thread_id without
correcting the 'NULL'able status of existing columns it fails.

We reorder the sql statements in the ugprade as follows:

ALTER TABLE slow_log MODIFY {columns} {new types} NOT NULL,....

As thread_id doesn't exist in the above statement it was removed from
the first ALTER TABLE statement to prevent failure.

Previous ALTER TABLE slow_log where moved later appending thread_id
and rows_affected, and also enforces the type of thread_id if it
was incorrectly like the now first ALTER STATEMENT slow_log used
to do.