fix(search): Allow to set empty type

Summary:
This small patch allows to set and empty type to the extended options, which will set the Any Type option (index 0).
It doesn't make any change in current code, since this case is not called, but it helps simplifying follow-up patches.

Test Plan:
No behavior changes with current code.
Calling setFacetsType(QString()) actually sets the first option (Any Type)

Reviewers: elvisangelaccio, Dolphin

Reviewed By: elvisangelaccio, Dolphin

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D25255