W3C - make image twig output empty ALT attribute when no ALT text is provided

Problem/Motivation

Images without an ALT attribute will fail basic W3C HTML validation.

Steps to reproduce

Add an image to a content page or slideshow ... and neglect to provide any ALT text.
Validate the HTML using W3C validator to see the error message about < img > must always have alt="" at the very least.

Proposed resolution

Modify the image twig to at least output an empty ALT attribute when no ALT text attribute is provided, and of course also output the ALT text when that is available.

Remaining tasks

None