NotifcationPopup: workaround crash in Qt when appending empty actions array

Summary:
Appending an empty array of actions to QQmlListModel triggers an assert
in Qt when built in debug mode, see https://codereview.qt-project.org/#/c/223985/
for details.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11519