fix(client): treat empty OPENAI_BASE_URL as unset and fall back to default

[NIK-TIGER-BILL]

Problem

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. As a result, the second if base_url is None check never fires, the default https://api.openai.com/v1 fallback is skipped, and the client tries to connect to an empty URL, producing a confusing APIConnectionError.

# Before (broken with OPENAI_BASE_URL="")
if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL")  # returns ""
if base_url is None:          # "" is not None — this branch is skipped!
    base_url = "https://api.openai.com/v1"

Solution

Convert the empty string to None using or None, so the fallback fires correctly:

if base_url is None:
    base_url = os.environ.get("OPENAI_BASE_URL") or None
if base_url is None:
    base_url = "https://api.openai.com/v1"

Applied to both the sync OpenAI client and the async AsyncOpenAI client.

Testing

• Syntax verified with Python AST parser
• Reproducer: export OPENAI_BASE_URL="" && python -c "from openai import OpenAI; OpenAI().models.list()" — previously raised APIConnectionError, now falls back to default endpoint

Fixes #2927