Akonadi: ignore remote ID if it's empty

Summary:
This fixes the error "only a resource can change the remote ID" when
editing an event in korganizer right away having created it.

Test Plan: as above

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D16917