MDL-37378 quiz restore: timecheckstate not in old backups.

We need to avoid a notice. If the value is missing, setting it to 0 is
fine. It will get sorted out the next time cron is run.

mod/quiz/backup/moodle2/restore_quiz_stepslib.php

@@ -306,7 +306,11 @@ protected function process_quiz_attempt($data) {
         $data->timestart = $this->apply_date_offset($data->timestart);
         $data->timefinish = $this->apply_date_offset($data->timefinish);
         $data->timemodified = $this->apply_date_offset($data->timemodified);
-        $data->timecheckstate = $this->apply_date_offset($data->timecheckstate);
+        if (!empty($data->timecheckstate)) {
+            $data->timecheckstate = $this->apply_date_offset($data->timecheckstate);
+        } else {
+            $data->timecheckstate = 0;
+        }
 
         // Deals with up-grading pre-2.3 back-ups to 2.3+.
         if (!isset($data->state)) {