MDL-27171 messages: fix bug in upgrade_populate_default_messaging_prefs
MDL-27171 messages: fix bug in upgrade_populate_default_messaging_prefs

isset check assumed that $defaultpreferences is the object

Signed-off-by: Ruslan Kabalin <87f18c11f8801edc4e4f2aa3256f0e5da60feb2b@luns.net.uk>

dilawar/moodle