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