fix(eslint-plugin-react-hooks): fix configs.flat properties typed as undefined in strict mode

[abhishekayu]

Fixes #36016

Summary

When using eslint-plugin-react-hooks with TypeScript strict mode, accessing configs.flat.recommended in eslint.config.ts throws:

The issue is that configs.flat was initialized as an empty object {} with a type cast. TypeScript doesn't know the properties exist until Object.assign runs later, so it treats them as possibly undefined in strict mode.

Two small changes to fix this:

• Use null as unknown as {...} instead of {} as {...} for the initial declaration
• Replace Object.assign(configs.flat, {...}) with a direct assignment configs.flat = {...}

Runtime behavior is identical - only the types change.

How did you test this change?

Checked that configs.flat.recommended and configs.flat['recommended-latest'] are no longer inferred as T | undefined in strict mode.

This now works without needing !:

import reactHooks from 'eslint-plugin-react-hooks';

export default [
  reactHooks.configs.flat.recommended, // no type error
];

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