Empty CategoryId in Item object after it's Created

Topic (or category item) url leads to home page in TOPIC.ADD email event in
case, when topic creation confirmation template is not used. This happens
because just after topic (any other category item) is created it's
CategoryId virtual field is not filled with current category (where topic
was added to), but this CategoryId field is used later to build link to this
topic in TOPIC.ADD email event.
To fix this you should set CategoryId field to $category_id (when passed) in
kCatDBItem::assignPrimaryCategory() method (only when $category_id is not
empty).