Offer skip condition

I created patch by #22

This feature is fundamental.

This patch needs a re-roll.

I rerolled the patch for 4.0.0-beta1

Should this be a feature in the next beta version? I'll have to rebase the merge request source branch, so anybody could do a review.

I restarted the issue fork branch based on current 4.x and applied the latest patch. The merge request is ready for a review again.

Is it possible to simply skip sending when the created newsletter body field is empty? This seems like it would be automatic, but evidently not.