Decorative image feature 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 → .)

Tested this using Standard profile and initially I was confused because it seemed as though I could reproduce it.

However, my issue was that I simply didn't click the green tick after toggling on "Decorative image" to save the changes. After doing that the image correctly gets an empty alt attribute. There's even a warning over the image if you don't do this and leave the alt text field blank.