popupMenu: Avoid open state reentrancy in dummy menus

Dummy menus may emit ::open-state-changed multiple times for
the same state. Avoid doing that so that the PopupMenuManager
is happy not having to handle reentrancy.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5064
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2178>