Dont add default valued date filters to json

Summary:
Commit 41d5013e5b7c ("SearchStore: Implement date filter queries") changed
the initialization/default filter from -1 to 0. Now, the
day/month/year filters are always added to the json data, although not
having any effect.

Test Plan:
run dolphin, select e.g. "Images"
yearFilter/monthFilter/dayFilter no longer appear in json'ed query

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20364