[applets/notifications] Remain visible after removing all notifications while...

[applets/notifications] Remain visible after removing all notifications while tray popup is pinned open

Summary:
When I pin a Notifications applet and then remove all notifications, the Notifications applet closes itself.

BUG: 421344

Reviewers: ngraham, plasma-devel, broulik

Reviewed By: ngraham, broulik

Subscribers: broulik

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29651