LPS-48404 don't index root classPK for KB articles

If present, the search portlet uses the root classPK to generate
URLs. KB uses the parent article as a root classPK, which makes the
search portlet redirect users to the wrong article (the parent).
LPS-48146 keep the property unset by default
LPS-48146 Hey guys, are you sure this works, setting it to blank? Won't that still default to 0? I'm looking at the logic from GetterUtil and it seems to me that's what it should do.