Allow components containing non-required images and no examples to be placed

Overview

Quoting @lauriii from #3501902-65: Adding the Image component results in a state considered invalid:

I'm getting this error when I try to place "Container" element from demo design system. It has an optional image without an example.
AssertionError: assert(\is_array($this->value)) in assert() (line 78 of /var/www/html/web/modules/contrib/experience_builder/src/PropSource/FallbackPropSource.php).

Quoting @longwave from #3501902-66: Adding the Image component results in a state considered invalid:

the problem with the demo Container is that it specifies an image without an example:

    # Don't put examples as this is used with and without images.
    image:
      $ref: json-schema-definitions://experience_builder.module/image
      type: object
      title: Image

but Experience Builder's image schema says the src property is required:

    "image": {
      "title": "image",
      "type":  "object",
      "required": ["src"],
      "properties": {
        "src": {
          "title": "Image URL",
          "$ref": "json-schema-definitions://experience_builder.module/image-uri"
        },

This causes a validation error because when you first add the component the image doesn't have a src. If an example was specified (as in the XB image component) then we use that as the fallback, but there is nothing to fall back to here either.

Proposed resolution

1. Allow GeneratedFieldExplicitInputUxComponentSourceBase::clientModelToInput() to skip resolving a value when a prop is not set.
2. Prevent computation of the default resolved value from crashing if no example is present.

User interface changes

None.