messageTray: Close SHOWING banner when its notification is destroyed

Currently only banners in the SHOWN state are hidden when the underlying
notification is destroyed, but if they are in the SHOWING state, they
remain visible. Because the 'notification' member has already been set
to null when the notification got destroyed, closing the banner by
clicking on the close button, will not do anything and clicking on the
notification itself will result in an error message. For notifications
without a timeout, i.e. critical ones, this will result in an
uncloseable notification.

This can happen if the program creating a critical notification
immediately closes it again, as might happen with power notifications
from gnome-settings-daemon in some situations.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4855
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2079>
(cherry picked from commit 2801d0bf)