applets/notifications: Fix url type safety in JobItem component

It was all wrong, down to the indentation 💀

Fixes the following error:

plasmashell: file:///usr/local/kde/usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/JobItem.qml:106:17: Unable to assign null to QUrl

Ironically, the line in question didn't do anything wrong. At least not from the point of view that bases on reasonable assumptions, like a url gadget typed property being an instance of url.