Preserve null NodeList forEach context

[OskarEichler]

Summary:

NodeList.forEach currently treats both null and undefined as the omitted-context fast path, so a strict callback receives undefined when the caller explicitly supplied null. Use the fast path only for undefined and cover the standards-compatible null receiver.

Fixes #58242.

Changelog:

[GENERAL] [FIXED] - Preserve an explicit null thisArg in NodeList.forEach callbacks.

Test Plan:

• Exact upstream focused suite: 12 existing tests passed and the new null-context regression failed.
• Fixed focused Fantom suite: 13/13 passed.
• Fresh Flow check: 0 errors.
• Targeted no-ignore ESLint, Prettier, and git diff --check passed.

Only the observably incorrect explicit-null context changes; omitted and undefined contexts are unchanged.