[BasicIndexingJob] Skip lookup of baloo document type for directories

Summary:
typesForMimeType("inode/directory") returns an empty list, so there is
no need to do the lookup.

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: poboiko

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21697