fix: handle empty OPENAI_BASE_URL env var by falling back to default

[giulio-leone]

Summary

Fixes #2927 — When OPENAI_BASE_URL is set to an empty string (e.g., export OPENAI_BASE_URL=""), the client fails with APIConnectionError instead of falling back to https://api.openai.com/v1.

Root Cause

if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL")  # returns "" not None
if base_url is None:  # "" is not None → fallback skipped
    base_url = f"https://api.openai.com/v1"

os.environ.get("OPENAI_BASE_URL") returns "" when the variable is set but empty. Since "" is not None, the default URL fallback is skipped.

Fix

base_url = os.environ.get("OPENAI_BASE_URL") or None

or None coerces empty strings (and other falsy values) to None, allowing the default fallback to activate. Applied to both OpenAI and AsyncOpenAI constructors.

Tests

Added 2 new test cases (test_base_url_env_empty_string) verifying both sync and async clients fall back to the default URL when OPENAI_BASE_URL="". All existing test_base_url_env tests continue to pass.

[giulio-leone]

Friendly ping — CI is green, tests pass, rebased on latest. Ready for review whenever convenient. Happy to address any feedback. 🙏