Treat empty OPENAI_BASE_URL as unset

[LittleChenLiya]

Summary

Treat an empty OPENAI_BASE_URL environment variable as unset so the client falls back to the default API endpoint.

Previously, OPENAI_BASE_URL="" bypassed the default fallback and produced an invalid empty base URL.

Changes

• normalize empty OPENAI_BASE_URL environment values to None
• apply the fix to both sync and async clients
• add sync and async regression tests covering the empty-env fallback

Testing

• python -m pytest tests/test_client.py -k "base_url_empty_env_falls_back_to_default or test_base_url_env" -n 0
• python -m pytest tests/test_module_client.py -n 0

Closes #2927