[3.11] gh-92986: Fix ast.unparse when ImportFrom.level is None (GH-92992) (GH-96593)

This doesn't happen naturally, but is allowed by the ASDL and compiler.
We don't want to change ASDL for backward compatibility reasons
(GH-57645, GH-92987)
(cherry picked from commit 200c9a8d)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>