gh-131421: Fix ASDL arguments expr* kw_defaults to expr?* kw_defaults

According to the docs:

> (https://docs.python.org/3.15/library/ast.html#ast.arguments)
> kw_defaults is a list of default values for keyword-only arguments. If one is None, the corresponding argument is required.

However, ASDL's description of `kw_defaults` is that it simply a
`expr*`, and not a `expr?*` as stated by the docs.

Author: Demonstrandum

Parser/Python.asdl

@@ -114,7 +114,7 @@ module Python
                     attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)
 
     arguments = (arg* posonlyargs, arg* args, arg? vararg, arg* kwonlyargs,
-                 expr* kw_defaults, arg? kwarg, expr* defaults)
+                 expr?* kw_defaults, arg? kwarg, expr* defaults)
 
     arg = (identifier arg, expr? annotation, string? type_comment)
            attributes (int lineno, int col_offset, int? end_lineno, int? end_col_offset)