[Background Finder] Create a valid invocation token

Summary: According to Qt docs, QUuid() creates a null UUID object.

Test Plan: N/A, plasmashell is very unstable with Qt 5.15.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27237