fix: handle undefined values in mergeRsbuildConfig

[chenjiahan]

Summary

Handle undefined values in mergeRsbuildConfig to ensure compatibility with the behavior prior to Rsbuild 1.6.

Related Links

resolve #6549

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR updates mergeRsbuildConfig to handle undefined values in its parameter list, restoring compatibility with behavior prior to Rsbuild 1.6.

Key changes:

• Updated function signature to accept (RsbuildConfig | undefined)[] instead of RsbuildConfig[]
• Added filtering logic to remove undefined values before processing
• Adjusted length check to return empty object when all values are undefined

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/mergeConfig.ts	Updated implementation to filter undefined values and handle empty config arrays
packages/core/tests/mergeConfig.test.ts	Added comprehensive test coverage for undefined value handling
website/docs/en/api/javascript-api/core.mdx	Updated English documentation with new type signature
website/docs/zh/api/javascript-api/core.mdx	Updated Chinese documentation with new type signature

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