fix: preserve OpenAI endpoint resolution for empty api_base

[wcqqq1214]

Fixes #10078. Missing or empty api_base values can leave the SDK endpoint empty instead of resolving it from environment variables.

Modifications / 改动点

Normalize api_base to None in both Azure and regular OpenAI client constructors. Explicit URLs are unchanged.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

Verified both actual provider branches with missing, empty, None, and explicit URL values: all 8 cases passed. Checked client.base_url with endpoint environment variables set; no API requests were sent or regression tests added.

ruff format ., ruff check ., and git diff --check passed.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Bug Fixes:

• Preserve environment-based endpoint resolution when OpenAI or Azure api_base is missing, empty, or otherwise unset while retaining explicit URLs.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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