lighter request on circle memberships

[ArtificialOwl]

linked to nextcloud/circles#1851

• add index to group_folders_groups.circle_id alone
• avoid useless LEFT JOIN on request (if lighter link to memberships circles#1851 applied)
• filters on empty circle_id to not search circles on 'group' groupfolders

new request:

SELECT
    `f`.`folder_id`,
[...]
    `a`.`permissions` AS `group_permissions`,
    `c`.`permissions` AS `permissions`
FROM
    `oc_group_folders` `f`
INNER JOIN `oc_group_folders_groups` `a` ON
    `f`.`folder_id` = `a`.`folder_id`
LEFT JOIN `oc_filecache` `c` ON
    (`c`.`name` = CONCAT(`f`.`folder_id`, '')) AND(`c`.`parent` = '185') AND(`c`.`storage` = '2')
INNER JOIN `oc_circles_membership` `w_j` ON
    (
        `w_j`.`single_id` = 'ysdjkgfhdslkjghalkj'
    ) AND(`w_j`.`circle_id` = `a`.`circle_id`)
WHERE
    `a`.`circle_id` <> ''

[provokateurin]

A test would be nice to ensure the intended results are returned from the query.