improve credentials null check

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.88%. Comparing base (81b66b7) to head (b3b8225).

Files with missing lines	Patch %	Lines
python/alibabacloud_tea_openapi/client.py	87.50%	2 Missing :warning:
ts/src/client.ts	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #168 +/- ## ========================================== + Coverage 81.13% 81.88% +0.75% ========================================== Files 5 5 Lines 3149 3197 +48 Branches 114 118 +4 ========================================== + Hits 2555 2618 +63 + Misses 518 503 -15 Partials 76 76 ``` | [Flag](https://app.codecov.io/gh/aliyun/darabonba-openapi/pull/168/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aliyun) | Coverage Δ | | |---|---|---| | [golang](https://app.codecov.io/gh/aliyun/darabonba-openapi/pull/168/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aliyun) | `76.88% <100.00%> (+0.44%)` | :arrow_up: | | [python](https://app.codecov.io/gh/aliyun/darabonba-openapi/pull/168/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aliyun) | `83.69% <87.50%> (+1.35%)` | :arrow_up: | | [typescript](https://app.codecov.io/gh/aliyun/darabonba-openapi/pull/168/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aliyun) | `89.22% <87.50%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aliyun#carryforward-flags-in-the-pull-request-comment) to find out more.

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