Author: Damyon Wiese
MDL-35963: Assignment - do not accept empty submissions
http://git.moodle.org/gw?p=moodle.git;a=commit;h=34b8f3a83dec2a92f918437013f1e8a2f430d09d
http://github.com/moodle/moodle/commit/34b8f3a83dec2a92f918437013f1e8a2f430d09d
+ 2.4 en [submissionempty,mod_assign]