Такое?
SELECT DISTINCT events.id, events.name as event_name, events.date_start, events.date_end, events.country_id, events.description, events.day_1, events.day_2, events.month_1, events.month_2, events.year_1, events.year_2, events.is_present, links.links, images.images, countries.countries, countries.country_ids, country_to_cluster.year_start FROM events
LEFT JOIN (SELECT countries.id, GROUP_CONCAT(countries.id) as country_ids, event_to_country.country_id as country_id, event_to_country.event_id, GROUP_CONCAT(countries.name) as countries FROM countries LEFT JOIN event_to_country ON countries.id = country_id GROUP BY event_to_country.event_id) as countries ON events.id = countries.event_id
LEFT JOIN (SELECT event_id, GROUP_CONCAT(link) as links FROM links GROUP BY event_id) AS links ON events.id = links.event_id
LEFT JOIN (SELECT event_id, GROUP_CONCAT(name) as images FROM images GROUP BY event_id) AS images ON events.id = images.event_id
LEFT JOIN keywords ON events.id = keywords.event_id
LEFT JOIN country_to_cluster ON country_to_cluster.country_id = events.country_id AND events.year_1 >= coalesce(country_to_cluster.year_start,events.year_1) AND events.year_1 <= coalesce(country_to_cluster.year_end,events.year_1)
WHERE (country_to_cluster.cluster_id = :cluster_id)
ORDER BY events.year_1 ASC
LIMIT 30
Это будет игнорировать null отдельно для каждого поля year_start и year_end. Если надо именно одновременный null, это будет чуть по другому