Incomplete test coverage for `ast.get_docstring`

[tomasr8]

Bug report

Bug description:

The ast.get_docstring is missing test coverage:

I'll send a PR with some additional tests ;)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-121671: Increase test coverage of ast.get_docstring #121674
• [3.12] gh-121671: Increase test coverage of ast.get_docstring (GH-121674) #121690
• [3.13] gh-121671: Increase test coverage of ast.get_docstring (GH-121674) #121691

[encukou]

Thank you for the fix!