layout: return None bounding box when no nodes found

[syvb]

The HTMLImageElement.prototype.width/height attributes should provide the rendered width/height of images that are being rendered, and the natural width/height of images that are not being rendered. In Servo, these attributes call bounding_content_box on the node, treating None as an indication that the element is not being rendered (and returning the natural width/height). But Layout 2020 returns a 0x0 bounding box for elements that are not being rendered, instead of None.

As a result:

let image = new Image();
image.src = "...";
image.onload = () => {
    const [width, height] = [image.width, image.height];
    // use width/height
};

always gets 0/0 for the width/height (instead of the natural width/height) in Layout 2020.

This PR makes it so requests to Layout 2020 for the bounding box of a node that isn't being rendered return None instead of a 0x0 bounding box, which matches the Layout 2013 behavior.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• There are tests for these changes (which already exist in WPT)