[EPS] Fix crash at app shutdown (being tried to persist clipboard image)

Summary:
Deny any capabilities when there is no QApp instance.

BUG: 397040

Test Plan:
Untested, as I do not experience the bug on my system and had no time to
invest into trying to.

Reviewers: zccrs, dfaure, pino

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15405