gh-153691: Fix empty-list handling in PyInitConfig_*StrList

[AayushMainali-Github]

Summary

PyInitConfig_GetStrList() and PyInitConfig_SetStrList() used malloc(n * sizeof(...)) even when n == 0. On platforms where malloc(0) returns NULL, that was treated as an out-of-memory failure, so a valid empty list (for example the default xoptions) incorrectly failed.

This change skips malloc(0) for empty lists:

• GetStrList returns length == 0 and items == NULL
• SetStrList stores {.length = 0, .items = NULL} (same as _PyWideStringList_INIT)

This matches the existing empty-list handling in _PyWideStringList_CopyEx().

Fixes #153691.

Test plan

• Reproduced the failure mode with LD_PRELOAD forcing malloc(0) → NULL
  • buggy path: GetStrList-style empty alloc returns -1
  • fixed path: returns 0 with items == NULL
• ./python -m test test_embed -v -m test_initconfig_get_api
• ./Programs/_testembed test_initconfig_get_api (with and without the preload)
• ./python -m test test_embed -v -m 'test_initconfig*'
• CI on GitHub Actions after the PR is opened

• Issue: test_initconfig_get_api fails on platforms where malloc(0) returns NULL #153691

[python-cla-bot]

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