??? That works fine, even with no records in items
for a given record in a
You will get NULL for the GROUP_CONCAT, of course.
Is the problem than you don't know what to do with the NULL in your PHP code?
If you want to provide a default value, in place of the NULL, just do this:
IFNULL( GROUP_CONCAT(items.id), '--none--' ) AS itemslist
FROM a LEFT JOIN items ON items.owner = a.username
GROUP BY a.username