[3.14] gh-138774: use `value` to `ast.unparse` code when `str` is `None` in `ast.Interpolation` (GH-139415) (#140503)

gh-138774: use `value` to `ast.unparse` code when `str` is `None` in `ast.Interpolation` (GH-139415)
(cherry picked from commit 4d0849426f4c6862e50658c4e35341ffb5ab288b)

Co-authored-by: George Ogden <38294960+George-Ogden@users.noreply.github.com>