[Notifications] Restore fallback to local URL when no place is found

This got removed in d82513ce as I thought a user would
always have a "Root" entry, leading to "Root/foo" paths.

However, if you hide a place, it will naturally not be considered for `closestItem`
resulting in the final `QUrl::toDisplayString` being hit, giving us a not so
nice `file:///` URL.