`_ssl._SSLContext` construction crashes when a bad build results in an empty `PY_SSL_DEFAULT_CIPHER_STRING` #define

[gpshead]

Bug report

What happened?

This doesn't come up in practice as nobody would ever have such a misbehaving build in a supported released config.

Not a security issue.

I stumbled upon this while working on my draft #116399 BoringSSL linkage branch in my own non-OpenSSL Linux environment.

Steps to reproduce

./configure --with-ssl-default-suites="" && make -j24
./python -m test test_ssl
... SIGSEGV ...

The desired result is a Python exception. Clearly nobody builds intentionally with the above flag as it would've crashed when they tried to use their build. I encountered this error a different way due to a non-functional ssl library config.

CPython versions tested on:

CPython main branch

Linked PRs

• gh-117310: Remove extra DECREF on "no ciphers" error path in _ssl._SSLContext constructor #117309
• [3.12] gh-117310: Remove extra DECREF on "no ciphers" error path in _ssl._SSLContext constructor (GH-117309) #117317
• [3.11] gh-117310: Remove extra DECREF on "no ciphers" error path in _ssl._SSLContext constructor (GH-117309) #117318