You need to choose what to do with it. You can either have a list of all a.username, and only matching itemslist within it (including NULL where no matching list exists), or no record of a.username if there is no matching itemslist.
I can't see what you are doing with a comma separating list that cannot be accommodated for using null instead. Personally I wouldn't use a group_concat, but that's just me. With the comma string, and a language like PHP, you'd execute an explode() call and it will return an array of each item. Problem is empty strings and nulls are considered a single character (that is, null), which can be exploded resulting in a single item of empty string.
$aList = array();
$aList = explode(',', $string);
Simple as that.