Avoid leaking a QObject in ExternalExtractor

Summary:
Passing a nullptr as a parent is fine, passing an untracked object
definitely is not.

Reviewers: Baloo, Frameworks, ngraham, poboiko, astippich

Reviewed By: Baloo, ngraham

Subscribers: dhaumann, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18891