Keep empty Json metadata docs out of DocumentData DB

Summary:
In case a document is marked for content indexing, but none of the
extractors returns any data, the Baloo::Result::finish() method will
serialize the data as an empty Json document, "{}".

Having no data for a document in the DB is no error, this is the case
already when content indexing is disabled or when the indexer has not
yet processed the document, so leave it out to save some space.

Reviewers: Baloo, Frameworks, astippich, ngraham, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20556