
Allow passing 'null' to setLevel and setLabel, so 'null' and 'undefined'
behave the same way.
Fixes a regression introduced by a42f7c23
, which caused parts of old
notifications to persist into new notifications, or elements being
incorrectly made visible
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2917>