[TermGenerator] Insert an empty position between independent terms

Summary:
Terms of a phrase have adjacent positions, but independent terms/phrases
should be spaced apart to not treat these as phrases.

Test Plan:
ctest -R querytest
(after D21864)

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21863