Make inviting_user in invitationsView nullable

[zenovich]

The situation where inviting_user (group_membership_changes.initiator_id) is null even for pending invitations having a corresponding (latest, with action='invitation_created') group membership change is possible and real. Actually, it's what we have in our dev database (and possibly the prod database too).

mysql> select at, group_id, member_id, type, action from group_pending_requests JOIN LATERAL (SELECT initiator_id, action FROM group_membership_changes WHERE group_membership_changes.group_id=group_pending_requests.group_id AND group_membership_changes.member_id=group_pending_requests.member_id ORDER BY at DESC LIMIT 1) AS latest_change ON latest_change.action='invitation_created' where type='invitation' AND action IS NOT NULL AND initiator_id IS NULL ORDER BY at;
+---------------------+---------------------+---------------------+------------+--------------------+
| at                  | group_id            | member_id           | type       | action             |
+---------------------+---------------------+---------------------+------------+--------------------+
| 2016-05-30 08:47:52 | 1061441261823291025 |  395414163411905857 | invitation | invitation_created |
| 2016-06-09 10:22:58 |  524710592747926617 |  758424489137954341 | invitation | invitation_created |
| 2016-09-12 16:45:07 |  265634762887890771 |   43494452251133038 | invitation | invitation_created |
| 2016-09-12 16:47:28 |  265634762887890771 |   64106307088691560 | invitation | invitation_created |
| 2017-06-02 23:23:28 |  351088315787638674 |  127087297316779357 | invitation | invitation_created |
| 2017-12-21 17:13:32 |  281477260571403056 |  694708648158149243 | invitation | invitation_created |
| 2018-09-21 21:41:51 |  162115639603334619 |  205404017817317958 | invitation | invitation_created |
| 2018-09-27 12:14:16 |  162115639603334619 |  590538280986825160 | invitation | invitation_created |
| 2018-10-10 14:36:33 |  940836791310406061 |  851979882668439468 | invitation | invitation_created |
| 2019-03-02 10:58:31 |  860323942498157720 | 1909944900306129841 | invitation | invitation_created |
| 2019-05-01 21:03:39 |  474667219360029565 | 1909944900306129841 | invitation | invitation_created |
| 2019-07-07 16:10:50 |   95223015275745886 |  261728824945523043 | invitation | invitation_created |
| 2019-07-10 15:10:15 |  125310545739285762 |  131067918929082935 | invitation | invitation_created |
| 2019-08-01 18:29:04 |  306882785614785617 |  796451867380488914 | invitation | invitation_created |
+---------------------+---------------------+---------------------+------------+--------------------+
14 rows in set (0.23 sec)

Because of 'ON DELETE SET NULL' in

  CONSTRAINT `fk_group_membership_changes_initiator_id_users_group_id` FOREIGN KEY (`initiator_id`) REFERENCES `users` (`group_id`) ON DELETE SET NULL,

new group_membership_changes rows having action='invitation_created' and initiator_id=NULL can be created from time to time.

Also, there are pending invitations that don't have corresponding (latest, with action='invitation_created') group membership changes (see #1123).

Here, in this PR, we address both situations. Also, when there is no corresponding group membership change, we use 'at' field from group_pending_requests instead of one from group_membership_changes.

Depends on #1122