MDL-58994 core_calendar: Coalesce event sort field

There are situations where we need to sort events on a field called
"timesort", but sometimes it is not set. So we can fall back to
"timestart" in this case.

calendar/classes/local/event/data_access/event_vault.php

@@ -164,7 +164,7 @@ public function get_events(
             $coursesfilter,
             $where,
             $params,
-            "e.timesort ASC, e.id ASC",
+            "COALESCE(e.timesort, e.timestart) ASC, e.id ASC",
             $offset,
             $limitnum,
             $ignorehidden