IDLE module-name completion crashes on a non-iterable __all__

Bug report

Bug description:

When the IDLE shell namespace has a non-iterable __all__ (for example __all__ = None, an easy typo), pressing Tab for module-name completion at an empty prefix raises an uncaught TypeError instead of showing completions.

autocomplete.AutoComplete.fetch_completions handles the ATTRS empty-prefix case with sorted(eval("__all__", namespace)) and no guard, while the sibling non-empty-prefix branch already wraps the equivalent sorted(entity.__all__) in try/except and returns empty lists on failure. Only the empty-prefix branch is affected.

Reproduction in the IDLE shell:

>>> __all__ = None
>>> # press Tab at an empty prefix
TypeError: 'NoneType' object is not iterable

A deterministic regression test that drives fetch_completions with a patched __main__.__all__ is included in the linked PR.

Suggested fix: guard the empty-prefix branch the same way the sibling branch is guarded, returning empty completion lists when __all__ cannot be evaluated or sorted.

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-153507
• gh-153527