[3.11] gh-93442: Add test for _Py_CAST(nullptr). (gh-93505)

[miss-islington]

(cherry picked from commit 713eb18)

Co-authored-by: Neil Schemenauer nas-github@arctrix.com

[cpython-cla-bot]

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[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

@nascheme and @corona10: Status check is done, and it's a failure ❌ .

[miss-islington]

Status check is done, and it's a success ✅ .