applets/digital-clock: give PlaceholderMessage checkmark icon

We use a checkmark icon for the notifications applet since having no
unread notifications is considered a good thing. In this busy modern
world, having no scheduled events on a particular day can be considered
in the same way, so let's use a checkmark icon here too.

applets/digital-clock/package/contents/ui/CalendarView.qml

@@ -480,6 +480,7 @@ PlasmaExtras.Representation {
 
                 visible: holidaysList.count == 0
 
+                iconName: "checkmark"
                 text: monthView.isToday(monthView.currentDate) ? i18n("No events for today")
                                                                : i18n("No events for this day");
             }