Update to the nvl2 function description

[liuyq-617]

Description

Please briefly describe the code changes in this pull request.

Checklist

Please check the items in the checklist if applicable.

• Is the user manual updated?
• Are the test cases passed and automated?
• Is there no significant decrease in test coverage?

[gemini-code-assist]

Summary of Changes

Hello @liuyq-617, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求主要旨在提高文档的准确性。具体来说，它更正了中文参考文档中 NVL2 函数的描述，确保用户了解当第一个表达式为空时，该函数正确的返回值行为。

Highlights

• 文档更正: 更正了 NVL2 函数的描述，以准确反映其在 expr1 为空时的返回值行为。
• 准确性提升: 更新了中文参考文档中 NVL2 函数的功能说明，确保用户理解其正确逻辑。

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[gemini-code-assist]

Code Review

This pull request correctly updates the description of the NVL2 function in the Chinese documentation (docs/zh/14-reference/03-taos-sql/22-function.md). The original description was incorrect, and this change fixes it. The addition of a newline at the end of the file is also a good practice.

While reviewing, I noticed that the English documentation (docs/en/14-reference/03-taos-sql/22-function.md) has the same error in the NVL2 function description on line 2321. It currently states: 'If expr1 is not null, return expr2, otherwise return expr1.', but it should be '...otherwise return expr3.'. It would be great to fix this as well to maintain consistency across all documentation.

Overall, this is a good and necessary documentation fix.