applets/notifications: ensure item exists before querying its properties

In NotificationItem.qml, we query a loader's item's implicit height only when the loader is active. But it's possible for the loader to be active and not yet have an item, since the item may take a moment to load. So let's guard that by also making sure the item is set before we query its properties.

Fixes this error:

Jun 07 08:00:47 Liberator plasmashell[227890]: file:///home/nate/kde/usr6/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/NotificationItem.qml:381:13: TypeError: Cannot read property 'implicitHeight' of null

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