[SNI] Fix race condition in item registration

Summary:
If StatusNotifierItem is registered and then immediately destroyed, it is possible that QDBusServiceWatcher will not emit the serviceUnregistered signal.
Add an additional check to avoid such situations.

BUG: 416652
FIXED-IN: 5.18.0

Test Plan: Telegram should not add empty items.

Reviewers: #plasma_workspaces, #plasma, davidedmundson

Reviewed By: #plasma_workspaces, #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27126