fix(client): treat empty OPENAI_BASE_URL as unset, falling back to default endpoint

[NIK-TIGER-BILL]

Description

When OPENAI_BASE_URL is set to an empty string (e.g. export OPENAI_BASE_URL=""), os.environ.get("OPENAI_BASE_URL") returns "" which is not None. This causes the second if base_url is None check to be skipped, so the empty string is passed as base_url to the HTTP client, resulting in confusing connection errors rather than falling back to the default endpoint.

Root Cause

# Before fix
if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL")  # returns "" if set to empty
if base_url is None:                               # "" is not None, skipped!
    base_url = f"https://api.openai.com/v1"

Fix

# After fix
if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL") or None  # "" becomes None
if base_url is None:
    base_url = f"https://api.openai.com/v1"               # fallback works correctly

This fix is applied to both OpenAI (sync) and AsyncOpenAI classes.

Changes

• src/openai/_client.py: Use or None to coerce empty string env var to None in both sync and async client constructors

Closes #2927