Fix warning

Summary:
Now if appIcon isn't defined, it will set the property as undefined.
file:///home/apol/devel/kde5/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/NotificationDelegate.qml:114: ReferenceError: appIcon is not defined

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16771