test: cover List empty state with permanent filters

[terminalchai]

Summary

• add a regression test for the List empty state when only a permanent filter prop is set
• confirm the custom empty component still renders when there are no results and no user-applied filters

How To Test

• corepack yarn test-unit-ci packages/ra-ui-materialui/src/list/List.spec.tsx
• corepack yarn exec prettier --check packages/ra-ui-materialui/src/list/List.spec.tsx

Notes

This does not change runtime behavior. It adds coverage for issue #8320, which appears to already be fixed in current master.

Closes #8320