[server] Do not try to create data offers without source

Summary: An internal drag is without data source. Still we tried to create offers.

Test Plan: This change makes the updated autotest in D15072 pass without errors.

Reviewers: KWin, hein

Reviewed By: hein

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15074