fix: BaseError@walk to return null when predicate fn not matches

[Alexsey]

As discussed here, the current implementation of BaseError@walk(fn?) has a bug when fn is provided and no error matches it

Current behavior: the deepest error will be returned
Expected behavior: null should be returned

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PR-Codex overview

Focus of the PR:

Fixing the BaseError.walk function to return null if the predicate callback is not satisfied.

Detailed summary:

• Fixed BaseError.walk to return null if the predicate callback is not satisfied.
• Updated the implementation of walk in src/errors/base.ts.
• Added a new test case for walk in src/errors/base.test.ts.

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