[drupalImage] Decorative image toggle doesn't add empty alt

There's explicit test coverage in HEAD to ensure this works correctly for decorative images

    // Ensure that the decorative image downcasts into empty alt attribute.
    $editor_dom = $this->getEditorDataAsDom();
    $decorative_img = $editor_dom->getElementsByTagName('img')->item(0);
    $this->assertTrue($decorative_img->hasAttribute('alt'));
    $this->assertEmpty($decorative_img->getAttribute('alt'));

(Added in #3222756: Allow using images from external source → .)