Search the database
SELECT * FROM `wp_postmeta` WHERE `meta_key` LIKE '%_oembed%' AND `meta_value` = '' ;Delete the first 500 values
DELETE FROM `wp_postmeta` WHERE `meta_key` LIKE '%_oembed%' AND `meta_value` = '' LIMIT 5000;
SELECT * FROM `wp_postmeta` WHERE `meta_key` LIKE '%_oembed%' AND `meta_value` = '' ;DELETE FROM `wp_postmeta` WHERE `meta_key` LIKE '%_oembed%' AND `meta_value` = '' LIMIT 5000;