Fix Matomo issue "doesn't have a default value"

fix_matomo_null.sql

# An upgrade of MariaDB broke our matomo installation
#  Error in Matomo (tracker): Error query: SQLSTATE[HY000]: General error: 
#  1364 Field 'visit_total_events' doesn't have a default value In query: INSERT INTO piwik_log_visit ...
#
# See https://github.com/matomo-org/matomo/issues/14799

ALTER TABLE piwik_log_visit MODIFY COLUMN visit_total_events SMALLINT(5) UNSIGNED NOT NULL DEFAULT 0;
ALTER TABLE piwik_log_visit MODIFY COLUMN config_os CHAR(3) NULL;
ALTER TABLE piwik_log_visit MODIFY COLUMN config_browser_name VARCHAR(10) NULL;
ALTER TABLE piwik_log_visit MODIFY COLUMN config_browser_version VARCHAR(20) NULL;
ALTER TABLE piwik_log_visit MODIFY COLUMN config_browser_engine VARCHAR(10) NULL;