Git commits in Moodle 3.9.3 by Simon Mok

commit 6de6fe9590fecff5ec478f007fe9150790dd2c03
Author: Simon Mok <simonmok@outlook.com>
Date:   Mon, 12 Oct 2020 21:43:48 +0800
Tag:    v3.9.3

MDL-68070 messaging: Fix error when personal notification is disabled