AST: ensure kw_defaults is always allocated for kw-only args (fix NULL deref)

[AZero13]

Bug report

Bug description:

has_kwonlydefaults can null-deref kw_defaults when the AST arguments node is constructed with kwonlyargs but a null kw_defaults.

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-142462: Ensure kw_defaults is always populated in AST arguments #142463