fix(client): treat empty OPENAI_BASE_URL env var as unset

[rmotgi1227]

Fixes #2927.

Summary

When OPENAI_BASE_URL is set to an empty string in the environment, os.environ.get("OPENAI_BASE_URL") returns "". The subsequent if base_url is None check is skipped, so base_url = "" is passed to httpx — which raises APIConnectionError: Connection error before any network request is made.

Root cause

if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL")
if base_url is None:          # ← empty string is not None, falls through
    base_url = "https://api.openai.com/v1"

Fix

Change is None → not base_url so both None and "" fall back to the default:

if not base_url:
    base_url = "https://api.openai.com/v1"

Applied to both OpenAI.__init__ (line 214) and AsyncOpenAI.__init__ (line 720).