[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