add validation for image source

[qmjose]

Check for src attribute being defined before setting it. This will help with <source> tags within <picture> tags to avoid the unnecessary src="null" on <source> tag that is required if the source element's parent is an <audio> and <video> element, but not allowed if the source element's parent is a <picture> element.

<picture>
  <source type="..." data-flickity-lazyload-srcset="...">
  ...
<picture>

<picture>
  <source type="..." srcset="..."  src="null">
<picture>

Based on MDN Documentation