fix(types): correct PathValueImpl type inference

[joseph0926]

Problem

In v7.64.0, commit 8d61561 changed PathValueImpl to support optional array fields,
However, the condition undefined extends T is too broad and interferes with TypeScript’s reverse type inference, so in some cases the inferred type ends up as never

ex)

const { field } = useController({ name: 'items', defaultValue: [] });
// error: Type 'string' is not assignable to type 'never'

Solution

// src/types/path/eager.ts

- : undefined extends T
+ : T extends undefined

This keeps the support for optional array fields introduced in PR #13057while restoring correct type inference.

Testing

• All existing tests continue to pass
• Added a type test for this issue in src/__typetest__/useController.test-d.ts

Related Issues

Fixes #13081
Related to PR #13057