Undefined can clear checked and value properties for custom elements

[objerke]

This pull requests makes it possible to reset the value and checked properties on a custom element, by passing undefined as a prop value. Tests are added with the new behavior.

This is a follow up on #2472 which slightly altered the behavior of #2465 in terms of undefined values. Currently an undefined value is never set for the value and checked properties.

it(`should clear existing 'value' property with ""`, () => {
    render(<test-component value={{ foo: 'bar' }} />, scratch);

    render(<test-component value={undefined} />, scratch);

    expect(scratch.querySelector('test-component').value).to.equal("");
});

We are using preact to render custom elements that uses properties to pass down data. The value can be cleared as a way to indicate an indeterminate state. I think it feels natural to allow the value and checked properties to function as any other property when it comes to custom elements.

[github-actions]

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