iCal, iCloud, Mac OS X: Empty "Home" and "Work" calendars may appear after upgrading

If you don't immediately migrate your iCal data in Migration Assistant after installing Mac OS X v10.5 Leopard or later, and you open and close Mail or the Calendar widget before opening iCal (even if you use Migration Assistant after opening Mail or the Calendar widget), empty "Home" and "Work" calendars may appear.