Git commits in Moodle 3.5.4 by Davo Smith

commit e35c7c2c50b836d9f03016aa61296c9a2d3ee685
Author: Davo Smith <davo.smith@synergy-learning.com>
Date:   Mon, 29 Oct 2018 17:26:58 +0800
Tag:    v3.5.4

MDL-63798 mod_assign: fix restore when group is missing