Optional object props with no value are now absent instead of `0`, `''` or `FALSE`

Before

When an optional property of an object-shaped prop had no value, Canvas substituted the data type's zero value: 0 for integers, '' for strings, FALSE for booleans. For example, an SVG image, whose dimensions the image toolkit cannot read, resolved to:

{"src": "/sites/default/files/logo.svg", "alt": "Logo", "width": 0, "height": 0}

After

• Optional unpopulated properties are omitted:

  {"src": "/sites/default/files/logo.svg", "alt": "Logo"}

  This is visible in inputs_resolved over the internal HTTP API and in the props a code component receives. SDCs use Twig templates and hence are unaffected: an absent variable prints as an empty string.

• Required SDC/code component props are unaffected: Canvas only maps a required prop onto a field property that is itself required, and an empty required property is still treated as inaccessible.

What to do

• A code component that read image.width || fallback keeps working. One that compared image.width === 0 must compare image.width === undefined instead.
• A code component that passed image.alt straight to an <img> now renders no alt attribute for an image that never had alt text, where it used to render alt="". Decide explicitly: alt={image.alt ?? ''} keeps the old behavior.