At least TagCurrentChanged can be triggered with a nullptr tag. That
means the current tag is reset to "none". We should thus handle nullptr
in all receivers of beginTagChange and endTagChange.

Change-Id: Ife558beca9fb1ed5ab246b76bbaab19c1c227e8a
Reviewed-by: Marco Benelli <marco.benelli@qt.io>
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>