[3.9] bpo-39728: Enum: fix duplicate `ValueError` (GH-22277) (GH-22282)

fix default `_missing_` to return `None` instead of raising a `ValueError`
Co-authored-by: Andrey Darascheka <andrei.daraschenka@leverx.com>
(cherry picked from commit c95ad7a91fbd7636f33a098d3b39964ab083bf49)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>
4 files changed