Change the default return value of `HTTPConnection.get_proxy_response_headers` from `{}` to `None`

[sobolevn]

See my reasoning here: #104248 (comment)

Linked PRs

• gh-105626: Change the default return value of HTTPConnection.get_proxy_response_headers #105628
• [3.12] gh-105626: Change the default return value of HTTPConnection.get_proxy_response_headers (GH-105628) #106738

[sunmy2019]

Indeed, None better represents what we have here.

[hauntsaninja]

Thank you!