[3.12] gh-126105: Fix crash in `ast` module, when `._fields` is delet… (#126132)

[3.12] gh-126105: Fix crash in `ast` module, when `._fields` is deleted (GH-126115)

Previously, if the `ast.AST._fields` attribute was deleted, attempts to create a new `as`t node would crash due to the assumption that `_fields` always had a non-NULL value. Now it has been fixed by adding an extra check to ensure that `_fields` does not have a NULL value (this can happen when you manually remove `_fields` attribute).
(cherry picked from commit b2eaa75b176e07730215d76d8dce4d63fb493391)

Co-authored-by: sobolevn <mail@sobolevn.me>