Notification of string changes

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


http://git.moodle.org/gw?p=moodle.git;a=commit;h=bf171078822c76fc0c0bea8ab3dbac2cae7a6454
http://github.com/moodle/moodle/commit/bf171078822c76fc0c0bea8ab3dbac2cae7a6454

+ 4.0 en [messageundeliveredbynotificationsettings,core_error]