[ModifiedFileIndexer] Correct time checks for new files

Summary:
In case baloo has just started, a new and immediately modified file can
end up in the ModifiedFileIndexer. In this case, the timestamps default
to 0. As a 0 timestamp also occasionally appears e.g. for files from
archives (tar, zip ...) these would be treated as unchanged and not be
added to the index.

Depends on D27766

Reviewers: Baloo, ngraham, astippich

Reviewed By: astippich

Subscribers: astippich, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D27767