Return 404 or member list when getting joined_members after leaving (#13374)

Signed-off-by: Andrew Doh <andrewddo@gmail.com>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Andrew Morgan <andrewm@element.io>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>